The Ford 6.8L is used in many class C motorhomes and is about 415 cubic inches.
As for comparing engine HP to a 3.3L diesel you will have a bunch more power from the larger gas engine, and it can run all day at 4,000 RPM climbing hills, ect, pulling a large trailer and such. The Ford F-450 motorhomes have a 14,050 GVWR (total weight limit on all 6 tires) and a 22,000 GCVWR (total of motorhome and what it is towing). So with a 13,000 curb weight, a F-450 based motorhome can have 1,000 pounds on the hitch, and be towing a trailer with a 9,000 pound total weight.
The small diesel engines used in Sprinter van based motorhomes are not rated to tow a lot more weight, than the basic GVWR of the motorhome itself.
Good luck,
Fred.