A travel trailer is not the answer, not with your weather. We camp in a 2009 Puma travel trailer and use it about 30-40 days per year, and the are not made for cold weather. Yes, they are easy to heat due to the small square footage, but they are not insulated well. If you buy a 4 season trailer, with a heated underbelly, larger furnace, and good insulation, you are gonna spend a fortune. Arctic Fox, Northwoods, and Holiday Rambler make some nice 4 season trailers. Go check the prices.
Also, if she has mobility problems, a travel trailer is not the easiest thing to get around in. Everything is compact.
Seriously, if you are set on putting her out there in a trailer, get a used 12X60 mobile home and have someone run a septic line, water, and at least 60 amp electric to it. Used mobile homes are very cheap. Look on your local Craigslist or the local paper.
Bob