How will getting in and out of the 5th wheel be any different (hard on your dog's back) than in and out of the tow vehicle? If it were my dog I would look into a dog ramp that can aid in getting in and out of both the 5th wheel and tow vehicle.
Mark
Mark & Renee
Working Border Collies: Nell (retired), Tally (retired), Grant (semi retired), Lee, Fern & Hattie
Duke & Penny (Anatolians) home guarding the flock
2001 Chevy Express 2500 Cargo (rolling kennel)
2007 Nash 22M