I have a 2006 Keystone Raptor (5th wheel toyhauler). I picked it specifically for the ability to use for snowmobiling.
The underside of the trailer (living area) is completely enclosed and insulated. The tanks are all heated. The frame is aluminum (not the main trailer frame, but the trailer "box" itself is). The garage floor is covered in "diamond plate" rubber.
Meltoff from the sled will just run down the floor and out the crack under the back door. Without wood framing, I'm not concerned about mold. Plus, when the trailer is stored, it sits at an angle so the water will run out even more easily, rather than stand.
The only thing I haven't figured out how to handle yet is another layer of protective rubber on the garage floor (and on the ramp door) to protect it from the carbides. I think I'll see if I can't find a roll of rubber material similiar to those truck bed mats, and have it cut to fit. Might get pricey, but it's better than a sliced up garage floor.
Another thing I want to do is insulate the underside beneath the garage. The living area is already done, but the garage area is open. It really isn't a big deal, but it'd be nice.
To answer the question about which trailers to buy... look for one with an aluminum structure instead of wood. Look at the garage floor and make sure it's well-sealed so that if water does any "seeping" there isn't anything to damage. Try to make sure the water lines and tanks are insulated if not heated too.
If you camp in really cold weather, get one WITHOUT any slide-outs. My Raptor only has one (living area), which I will keep retracted at night to reduce the amount of area that has to be kept heated. With such a tall roof, the furnace has its work cut out for it.
Try to get one with extra storage space for batteries. If you can find a way to heat the battery compartment, do so (I need to do this too), since batteries don't hold a charge well in cold weather.
A local guy, who will hopefully see this thread and respond, bought a regular trailer (not a toyhauler -- he carries his sled on a sled deck and then tows the trailer), specifically without slides, and as small as he could get while still having enough ammenities. He then removed most of the interior, cut and installed extra insulation anywhere he could fit it. He cut holes in the kickpanel areas of the cabinets to get more warm air in there to keep the water lines from freezing. I may have to do that to my Raptor sometime.
Rob