A trip through france and spain will be a great way of testing and fine tuning your "operating procedures".
I have been using
The UK Camp Site for Tent and Caravan Campers in the UK for information on good routes and stop overs all the way through france and spain. It's well worth a look. Be aware that allot of the "route aires" (camp sites at motor-way service stations) in France are not the safest places to camp even though they very cheap. I prefer to stay at sites close to my chosen route.
A good tip regarding traveling in the land rover with kids is to set off early in the morning. After the first 2 hours stop for a breakfast cook up, then do another couple of hours. You can then be in your next site by 11am,ready to do some chilling out or exploring. My kids seem able to do this routine without too much resistance. A few years back i tried to do the full french Atlantic coast in one go from UK to Biarritz. This required at least 2 days of recovery and is not recommended.
What sort of trailer have you scored????