I have a Grand Design Momentum 25G that we tow with a 3/4 ton Duramax, bought just before Covid hit. It's a bumper pull toyhauler so I can't comment on the Imagine line.
I can say we have been very happy down into the teens with it. Haven't been any colder but all the tanks and piping beneath are covered/insulated. If the Imagine is similarly equipped I think you will be happy with the cold weather performance.
Grand Design quality before Covid was better than average, and the interiors are more modern than other makers' offerings. Owners forums indicate GC quality suffered quite a bit during Covid, and I just don't know if they have their act back together yet.
Your experience will very much be a function of your local dealer's competence. Honestly, I have seen/heard/experienced the knuckle draggers working in those places and I'll do my own service thank you very much. I don't care if it's under warranty, I'll fix it now and use it this weekend instead of having it sitting at the dealer waiting on parts or waiting to fix it maybe right the second time in a month.
You just have to accept that doing a lot of your own maintenance comes with the territory on trailers.
Also plan on doing some customizing. The quality of your camping experience can be greatly enhanced by upgrading faucets and shower heads, mattresses, install a keyless lock, larger propane tanks, extra battery, a water filter, solar, etc etc.