When I was an RV transporter in the mid-2000s I spent three years on the highways all over the US and CN and in a lot of truck stops. As a lifetime RVer I like trucks and always pay attention to them so I've seen many of them towing large 5th wheel trailers.
One of the three companies I worked for was owned by an old retired trucker who owned an older Freightliner MDT with a plush sleeper cab and also a retired Class 7 tractor that had been owned by a trucking company that pulled doble trailers. I delivered an trailer somewhere in California for him using his Freightliner and drove his Class 7 tractor once when we went to lunch. i had never driven an OTR tractor and was unfamiliar with shifting a non-synchro 10 speed. He was an axxhole and enjoyed watching me struggle to match engine rpm and road speed to change gears. I wasn't very good at it.
I was surprised to learn the hitch plate and jaws on a Class 7 or Class 8 tractor fit the kingpin of an ordinary 5th wheel RV and is approximately the correct height. A retired HD tractor needs only an RV 7 pin receptacle to connect to and tow a 5th wheel . . . but a Class 8 tractor is a lot of truck to drive around when not towing. A Class 7 single-rear axle would be a better choice. The frame rails behind the cab and the driveshaft could be cut to shorten by a truck outfitter at a modest cost to make it more practical. The major drawback would be cost of operation and maintenance. Big trucking companies that own fleets of trucks put hundreds of thousands of miles on their tractors before they recycle them. Repairs could break you.
I like the newer MDTs. When we moved from west Texas to WV in 2018 we moved ourselves by hiring laborers on both ends to load and unload and I rented and drove three separate Penske 26' rental box trucks. I drove both Freightliners and Navistar trucks but all three had a Cummins 6.7 liter turbodiesel engine. I think the transmissions were Aisin 6 speed automatics because shifting and gear ranges were identical to my 2008 Ram C&C 3500.
The trucks accelerated strong, even fully loaded, were reasonably economical, and a pleasure to drive. They were equipped with easy power steering, very strong anti-lock air brakes, HVAC, and a basic Am/FM radio. I think they also had air ride suspension (not sure now) but not air ride seats. Also, excellent large rear-view mirrors and basic manual roll-up side windows. The trucks are Class 6 with 26,000 lb. GVWRs so renters didn't need a CDL but the same truck could have a 33,000 lb. GVWR if commercial. All three Penske rentals had low (under 100k) miles on them. I don't know how many miles Penske puts on their rentals before they recycle them but Penske dealers could probably tell you.
If I were planning to embark on a new RV retirement lifestyle with a big 5th wheel trailer I would search for a used but well-maintained Penske box truck. A Garmin GPS and an upgraded sound system could make it more comfortable. Anyone can drive one - wives included. Even with high miles one of those trucks would give you years of good service. Even factoring some repairs, replacements, and maintenance into the cost a recycled box truck could give you years of service cheaper than a new Class 4 Ford (grimace). Again, I would have a truck outfitter cut the frame rails and driveshaft to shorten the extremely long wheelbase.
