I would get the trailer; and if Mom doesn't want to go, I'd say, "see ya in a few days!" and I'd go camping. ;)
But I want to make sure you're realistic about the shower thing. That T@b has a 10 gallon fresh water tank. That's the same as my former 16' KZ trailer. The water outlet on my tank was not right at the bottom, so a gallon or more always stayed in the tank; the pump could not draw it out. So, 8.5 or 9 useable gallons at most. The point is, your showers will be very skimpy.
Next problem is, in Idaho winters I doubt you will want to have any water in the T@b's tank at all. It would freeze.
On the plus side, the Subaru should handle well on slick roads... possibly better than a pickup would.