Originally Posted by 87MH
The huge moment arm created by the longer MoHo's and any side thrust created by a toad will quickly get you into trouble - even with a good brake system and controller on the toad.
87MH has the right of it. To see the effect of moment arm, look at the Airstream Interstate— not to suggest that the OP to buy one, just for illustration purposes.
A non-EXT Interstate has a rated towing capacity of 7500 pounds.
An EXT Interstate— exact same GVWR, exact same GCWR, exact same hitch receiver installed, but a body 16 inches longer, only has a rated towing capacity of 5000 pounds.
Less than a foot and a half of extra moment arm reduces the rated towing capacity by more than a ton, with all other things actually
To translate that to full-size motorhome terms, the motorhome with the highest towing capacity will probably be the one with the shortest rear overhang from rear axle to rear bumper, all other things being equal, just as with the Interstates.
Of course, length of rear overhang is not the only factor, but it's a very important factor in determining towing capacity.
The OP can't see a capacity posted over the driver's seat of a motorhome he hasn't yet bought, but he should be able to go to the Airstream webpage and freely download Adobe PDF files of the owner's manuals for any model and year of motorhome, and look up the towing capacities listed in the manuals.