I see a different spin. Your wife has no intention of driving the truck and 5er to the cabin. She is simply telling you that this is a non-negotiable weekend - drive her and the kids or else. If she has never bothered to drive it before, then she's not seriously planning on doing it now - she's using it as leverage against you (which means, you're going to go on this weekend trip).
In my mind, the idea of "forbidding" a spouse to drive a family vehicle/RV, regardless of their skill, is just wrong - to me, that's not a relationship worth having if one person feels it's OK to command the other. Discussing the issues of safely towing the 5er, hitching, etc., is a legitimate concern and can be had without using words like "forbid".
My solution is to do as the others suggest, get a campground site for at least one day.
As for the issue with organization, I sympathize. I like to have a basic schedule and I hate waiting for those who fly by the seat of their pants. My solution? I simply say "I'll be heading to the campsite at this time, and plan on doing X (whether a mealtime, sightseeing, lake-time, etc.) about this time. If you'd like to join me, please do so." Then I do as I have planned. If they really want to spend time with me, they will fit into my schedule. If they don't then at least I'm not sitting around all day waiting for them only to have wasted MY day while they did whatever they wanted.
