Two horses plus the trailer would probably weigh somewhere in the rough neighborhood of 5000 pounds, or perhaps more. Obviously it depends on the size of the horses, among other things. I'm guessing with two horses in a three horse trailer the tongue weight is going to be relatively on the high side (greater than 10%), and with the leverage from the rear overhang on the motorhome you're quite possibly adding 1000 pounds or so of weight to the rear axle. I suspect it would result in exceeding the rear GAWR, the GVWR, and/or the GCWR, and possibly all three; even if they aren't exceeded, you'd be very very near the limits, and that's probably not what you want for long-distance towing.
Note that some motorhomes have reduced tongue weight limits (compared to the trailer weight limits) due to the frame extensions and their construction. Partly, too, this is because many people use them for towing cars four down or on a dolly, both of which have little to no tongue weight.
(Backing a shortish trailer behind a longish vehicle with a long rear overhang is not exactly the easiest trailer handling job in the world, as well...though I guess a three horse trailer isn't super short.)