FIRST OF ALL - DON'T WORRY!! We all had to start somewhere, and many of us on here were just as nervous the first time. Once you come back you'll soon be booking again, and wondering why you were so worried.
The ports will be busy - but they are well organised, and you won't have any problems. Just listen to instructions, and you'll be pointed in the right direction for your ferry. When loading on to the boat there's often a ramp, which can catch your jockey wheel, so go very slowly, and don't worry about holding people up. On the other side when you first come off everyone will be going the same way, so just follow the crowds until you've got through the (usually unoccupied)passport control and 'customs' posts - then keep a sharp eye out for the signs for the route you want. Generally French roads are signposted by destination, rather than route number.
Roads on the other side are less busy than here - just remember 'keep your bum in the gutter' and you'll know which way to come out of junctions, go around roundabouts, etc. Have your passengers remind you to drive on the right every time you come out of a junction.
Fill up with petrol on the British side if this is your first trip, then you won't have to worry about getting fuel until you're a bit more used to driving. Diesel is Gazoil, or Gazole, Unleaded is Sans Plomb, and 98 is Super Sans Plomb. Fuel is usually cheaper in supermarkets but for your first trip fill up whenever you see a convenient petrol station on your side of your route. Drive in slowly, eyeing up the pumps and exits, and make sure you can get car and caravan out of the exit again, as some (not usually the ones on main routes)can be tight. Those fuel stations which state 'Auto 24hr' are usually unmanned and you can buy fuel in them with SOME British chip and pin cards, but not all, and not every station, so for this trip don't risk it.
Take some money with you in Euros, and pay for road tolls, fuel, first day spending, etc., with this. Then use your normal Bank Cards in the ATMs to draw out further cash - instructions are offered in English, just select the British Flag. Credit cards (if you have one) can be used for shopping in supermarkets, filling up with fuel after your first day or two, etc., and usually you get a better rate of exchange. It's worth opening Nationwide Credit Card and Debit card accounts as with these you don't pay a transaction charge each time you use the card - and you get a good exchange rate. Don't think about Traveller's Cheques - they are a pain in the .....!
Make sure you've got High Viz vests for yourself and your passengers, plus warning triangles. Strictly speaking, when towing you need three (sorry) - one to put in front of the car, and behind the van if you're still attached, and then if the car has to be driven or towed away without the van, another to put in front of the van which is to be left behind, keeping one in the car in case you should have another breakdown! Don't forget headlight adjusters too, if you intend driving in the dark.
Can't help with the timings, for your route, but try one of the route planners, such as viamichelin.com, or mappy.fr - which will calculate your route, (make sure you select 'with caravan'), give you an idea how much fuel you will use and how much road tolls might be.
Have a great trip - and don't forget to report back on how it went - it's a great help for first-timers if they see that someone was nervous, and yet made it there and back again!