After getting my new TT, I took my dog in it every day for afew minutes. Gave him some treats in there. Left him in there wwhile I worked around outside, spent time with him in there, even took a nap in there with him. When we started out he was nervous about everything in the camper, the sway when walking, the noises in and out. After a few days, he was excited and happy just to come out in the camper.
Acclimate them prior to going camping, make them enjoy the experience. Give them extra treats in the camper, give them new chew toys. Make sure they view the camper as a fun place, not a place they're going just to be left in. They'll view it as a punishment, and then get destructive.
If they are crate trained, and enjoy their crates, bring the crates.