I suspect getting a canoe in through the back window would be tricky. The openings aren't that huge typically. (On my coachmen, the opening of the exit window is 20" high and 28" wide.) A canoe is also going to extend into the motorhome quite a bit, and probably block the bathroom and/or kitchen access while en route, assuming you can get it inside.
Typical class C trailer hitches are limited at most to 500 pounds tongue weight, though you really need to find out details for the particular unit you're looking at—they can vary quite a bit. Assuming you're thinking of a motorized scooter, I would not really recommend carrying one on a hitch-mounted luggage rack; those racks apply stresses to the hitch that are rather different than a trailer (greater lever arm length) and often are not all that stable in terms of lateral rocking. Getting the scooter on and off the rack wouldn't be too easy, either. Putting several hundred pounds at the back is also going to affect your weight and balance some, which may or may not be a problem. (Very roughly speaking, the scooter would probably be about as far from the back axle as the front axle, and so every pound of scooter would load the back axle two pounds and unload the front axle one pound.)
There are some semi-custom designs with additional supports and ramps that are sturdier, which would help with some of the problems, but any weight and balance issues remain the same.
A small trailer might be more practical, though of course it's also more of an annoyance when traveling.
