Tom,
I appreciate all you are saying and hope it all works for you. I totally agree with your comments regarding speeding caravans, there are a lot of so called sensible people out there who take a lot of chances, with other peoples lives, as well as their own.
I don't know what the payload of the Dethleffs is but I am sure you will be weighing it carefully. Perhaps looking at one of the lighweight awnings would help to reduce weight, and, as we are finding out ourselves, is it necessary to travel with a hamper full of food? We usually end up bringing most of it home with us, so this year, we have decided to ditch most of it. There are supermarkets virtually everywhere so we can provision on a one, or 2 day basis happily. Similarly I have cut down to 1 gas cylinder as I now find that with electric on most sites, one cylinder lasts most of the season. I also carry a Camping Gaz 907 bottle both for the BBQ and as back up. If you have kids with you then obviously some of these ideas will not work, but if you are a couple like us it is amazing the unused 'c..p' we all tend to carry.
My Volvo is the S80, I did look at the V70 but felt that the estate was not going to be used all the time so the better room and comfort of the saloon was to be preferred. And i can swing down the back seat if i need to. However, the downside is that I am having to fork out for the Nivomats to get the better towing suspension. Cannot win them all. Good luck and if you are interested there is an excellent Volvo Owners Club with a lot of good info, and discounts.