hi. this is my first post here but ive been lurking for a while so I'll open with a longish post...

intro
Ive done a lot of touring in europe with my partner. to give you an bit of an idea, we've done 7 seperate trips in the space of 3 years on several different biking setups. first off it was second hand MTBs with rear panniers and top-of-rack bag only, then we hired dutch 3 speeds and had the same as previous setup plus a small bar bag each. then used that bag setup on another trip with hired mtbs. and another one on hired mtbs and 1 trailer, no other bags at all. then we bought decent touring bikes and trailers. we've done trips in England, Scotland, Wales, Norway, Sweden, The Netherlands, France, Germany, Slovakia, Hungary, Romania and Bulgaria.
Overnighting
we spent probably 80% of the time in our tent, and almost every time we were in a designated campground. you can free camp if you game, and we did in Norway and Scotland but I felt too exposed and invasive to try it in other countries. Scotland and Norway (and a few others) have differing 'rights of access' which mean you can pitch a tent in various places for free and noone will bother you. AFAIK France doesn't have this but I've talked to people that did free(stealth) camp. You've got to be quite sneaky, only setting up after dark and moving early and generally hiding etc, which felt wrong to us. We've spent a night in a police compound in our tent, using the prisoners toilet - but it was a lot safer than dealing with the crazy Romanian dogs that unrelentingly chased us...
We'd happily roll around until we found a cheap hotel - ask for prices and see the room and tell them you've got bikes. Stowed our bikes in a broken lift, stored everything in the unused lower floor pub, trucked everything through a nice hotel to hang it all off a balcony... People are surprisingly accommodating to a pair of tired cyclists. but mostly it was easy camp ground stuff. Lots of places will give you a discount because you're on bikes, as they seem to budget for a car on each site, the campground might even have a separate cycling patch - this is really common in the Netherlands.
Distance budget
on a bike tour in 3 weeks, i would budget covering about 700 km. my experiences and tourers we've met on the way all seemed to be stuck at 10-12km per hour as a trip average. This seems very low if you're a road rider, but this includes having a few breaks and getting off the bikes to look at/do things during the day - buy food, find the camp ground at the end of every day, and get around to washing your riding shorts.... we did do a few 100km days, and I met someone that did a 12 hour 205km day - but he was a powerful guy with a huge unceasing tailwind and zero hills. he was very proud of his day. mostly we did 70km a day - if you want to do anything else other than cycling, you won't be riding every day - we never did any more than 6 days straight and then would have 2 days off the bikes, and you might find that you just can't be bothered today, so you have spend the day in the pub.....
Food related
You'll be able to buy food easily, ever day in france, just watch out for the weekends, you can't carry much though so you'll find yourself in the shops a lot. Unless you just eat rice like one guy we met. but it is france, the food will be fantastic, so you'll need to eat it - id plan on having one entire bag full of food and still expect to get random picnics of jarred stuff you find in the little stores that dot the wayside.
Gear related
I would get a very comprehensive road atlas. I have 3 that include cycle ways - either before you go, or before you get on the bike over there. there's nothing worse than finding your stuck on a freeway because the nice divided road turned into an on-ramp! we got on freeways twice, one we could backtrack a few km, there other time this freeway was the ONLY south going road from the city for about 25km due to a large river... and I got a puncture.. hurrah!
I also have gps with all of Europes roads, I wouldn't ditch the real map though, the gps will never give you enough context on the tiny screens they have but they are damn useful for for showing you the immediate situation, mine also did turn by turn navigation - which was a nice backup if things went badly
get some very good wet weather clothing, it
will rain, and there's no way to ruin the day faster than a wet bum, potentially giving you the worst chaffing you'll ever experience. Even very good goretex will still get you wet as your hands and face will be bringing water in after all, so you can get by with rubber/plastic waterproofs. we did 3 trips with very cheap plastic trousers - they pack down to nothing. you dont want to ride in any waterproof legs for long. unless its warm enough that you'll dry fast of course.
mudguards are required aswell IMO, riding all day with your tires flicking road crap at you is not fun - and you'll get hot tar stuck in the most annoying places, and then theres the rain, and grotty oil slicked puddles.
water
proof panniers dont exist. Put good plastic bin liners in your panniers. you can get great waterpoof sacks, but its all adding weight and they are not cheap - especially when you need at least 4. I would use a good kayaking roll bag for the top rack bag.
don't expect the camp grounds to have cooking facilities - bbq, hot plates etc. this is an australian invention Im sure. Bring your own cooker and gas - if your free camping you'll have this anyway. keep your matches/lighter in a ziplock little bag.
I wouldn't ride into paris on a bike. ride somewhere close and catch the train - Versailles is easier to get into, has at least one nice campground and isn't a nightmare to navigate on a bike with gear. avoid the subway with your bike, my god that is hard work. Everyone hates you if you have a bike with gear and you're holding up the queues down there.
you'll be in peak cycling touring season, so expect a hell of a lot of company on cycle lanes,
you've said you want panniers, (or you have them) but I love my trailers so here goes. All your stuff goes in one place, you dont have to keep looking through all the bags, or trying to build a system that works - especially if its raining and you can't find the pegs or its freezing over night and you can't find your socks. the bike rides like a normal bike, not a bloated oil tanker. they can be water proof as you can buy a good plastic box designed for bad weather use with strong latches, water seals etc. a trailer and box will weigh a lot more than a bunch of pannieres, but a trailer and 1 huge kayaking waterproof sack will pretty good. Lastly, you can LOCK a plastic box, you will be leaving your bike on their own, and there's always the chance that some scumbag will want your stuff
I think thats about it, its a bit of an essay, hope it helps though. cycling touring is the greatest way to travel,