Tomer, I run the dunes a lot up here in New England...
Unfortunately, you're asking the wrong person when it comes to cleaning a truck.

? After my bi-weekly duning adventures, I only wash it if I have the time, and if it's actually that dirty enough.
I don't run in any salt water, but I'm guessing it would probably be a good idea to hit the undercariage with a powerwasher fairly soon after you're done. ?I can't help but wonder if a few salt water trips are as bad as as salt-filled winter in the snow-belt though. (I don't clean the Av at all in the winter) ?
I'm VERY jealous that you guys get to run off road bikes out there. ?Over this way, you venture off the trail and you stand the chance of being shot. (by eco nuts, not park rangers!) ?It's what we got though, so you have to respect it.