I suspect it's a personality thing. Our cat has been an indoor kitty her entire life, but when we moved across the country she had no choice but to become a "camping kitty." We have a TT so she rides in a carrier in the truck when we're on the road, and we carry her (in the carrier) between truck & trailer when we get settled in camp, and sometimes when we have lunch in the TT at a rest area enroute. For the entire move, and for subsequent years' camping trips, she meowed the entire time in the truck; fortunately no wailing or screaming, just meow - meow - meow - meow, for hours. She still does it periodically, but over the last 3 years of camping trips (3 weeks or less duration) she has gotten more used to it. It seems to help that we elevate her carrier in the truck (putting a large cushion under it) so she can see out; she seems happiest on 2-lane roads with lots of trees and/or green hills to see (go figure). In other words, she is getting used to the truck but still protests when we get her ready to go, and protests some of the time on the road. She loves being in the camper - no problems other than expressing her feelings by missing the litter box sometimes :R .
We got a larger carrier than usual for her, actually made for small dogs, so she has room to sit up, turn around, move from one side to the other, etc., and we don't use it to take her to the vet. (So far haven't had to do that on a trip!)
I suspect your two cats will be pretty much the same as they are now when riding in the truck. It's possible they might react differently in a MH. No way to know until you actually get underway.
We carry ours in the cab, in carriers. The new cats have almost spent more time traveling with us then at home.
They still hate travel days. But they like the new channels on Cat-TV (windows) when we get where we are going.
I guess that sums our experience up pretty well too. :)