Apart from the van, I've got a trailable yacht. Boat trailers are commonly dunked in saltwater past the bearings twice per trip. This definitely takes its toll the bearings and brakes, though they won't rust away from just one immersion. We commonly fit bearing buddies to our trailer hubs, and you should regularly check they are packed with grease. I'm not sure how the grease in the caravan wheels is retained, but presumably its fairly airtight otherwise it would leak out while driving?
A few tips I have from boats, that may apply to caravans:
Firstly if you drive in while the bearings are hot, they will cool rapidly under the water, and be more likely to suck water in. If they are very hot, and your are concerned it may be a good idea to wait before going through salt water.
Secondly you can get a quick check check on how healthy your bearings are by jacking the wheel off the ground and checking it spins freely with your hands. If it feels at all like its grinding or spins unevenly it may be time to look at the bearing!
Having salt water sit on trailers kills them, so look for spots where it could pool or get trapped, rather than running off once you are out of the water. Having good galvanising helps, though in the end everything will be eaten away by salt if you give it enough time.
Washing away salt water with fresh helps out a lot as well. Fresh water is a lot less corrosive than salt. Be careful that a lot of lakes and rivers are not "fresh water" like you get out of a tap, but are brackish. It helps to wash this off as well if you get the chance.
