where can a cat ride while travelling
Note that a 5th wheel is an often un-shock-absorbered wooden or aluminum box on the freeway behind a Carbon Monoxide generator. Having a loved person or animal in it increases their risk from the crazies and other dangers out there, as well as sending them airborne every time it hits a bump and increasing their chances of CO poisoning.
When the cats went from 2 to 7, we converted the underbed storage area to a 'cat house'. They seemed ok with that, since it was over the truck bed and seemed to benefit from the truck shocks. We tried a wireless baby monitor, but the backgound noise made that useless. The next step would have been a video system. but that is no longer an issue with the motorhome.
If you have a person riding in the 5ver, make sure you have communications, and they should stick to the part of the trailer over the truck bed as much as possible. Check with the laws too; in some places it is just not allowed. It would be a real good idea to have a CO detector in there too.