I doubt if compared like for like there would be much difference in maintenance and fuel costs for a MH or truck pulling a similar sized 5th wheel; if starting out with near new vehicles/RVs.
Many RV'ers choose a MH for ease of use, but even that has changed...in the past 5th wheels were not equipped with the same time/labor saving accessories as MHs. Auto levelers, power awnings, centralized sewer and freshwater setups, retractable power cords...etc. Today you can get a 5th wheel that is equally equipped as the MHs are.
We chose a 5th wheel for two reasons... more livable space and more flexible layouts. If you compare a 40 foot MH to a 40 foot 5th wheel the 5th wheel will have at least 3 feet more of useable living space. MH dash areas take away from 2 to 4 feet of useable living space. That 2 to 3 feet can mean an extra closet/ Washer Dryer cabinet or longer counters and cabinets in the kitchen or bathroom.
5th wheels can have living rooms or bedrooms in the front and or rear. MHs almost always have the living room in the front due to the driving area.