Gee we have a 5th wheel and I use a Toyota Avalon as my daily driver, the pickup sits until we need to tow the 5th wheel.
So I'm guessing you mean once you get setup at an RV Park you would have to use the HD Truck to get around town, sight see, run errands...etc. I enjoy driving my C4500 MDT around town. It's comfortable, the ride is a little rough but acceptable for 1 to 300 mile trips.
If you compare a 40ft 5th wheel to a 40ft MH you lose 3 to 6 feet of living space in the MH due to the driving area. That extra 3 feet or so allows for more counter/storage space in the 5th wheel.
We bought a 38ft 5th wheel instead of the 34ft model because the 38ft had a Washer Dryer area and the rear counter was 9 inches deeper than the 34ft model. The extra 4 feet in length provided more counter and storage space plus a W/D.
I'm thinking the designers view MHs as travel from A to Z (long distance) and stay for short periods of time. 5th wheel owners travel from A to B (shorter distance)and stay for longer periods of time or even as a permanent residence.
By the way most of the RV'ers we have met through the years, do very little cooking in the RV. They go out to eat, use prepared meals/microwave, snacks, or use an outdoor grill. We talked to many who have never used the stove and or oven in their RV.