I know this is a little late in the game, but I owned a Hallmark Guanella for 5-6 years, and I spent well over 400 nights in it. I live in Montana and used it year round. I spent a week in -10 to -20 temps during a blizzard up in the Boulder mountains during elk season and other than the water pump freezing, it was a great camper. All I had to do was open the floor where the water pump was housed and eventually it thawed out.
Just keep it vented to avoid excessive moisture and they are good to go.
I miss that camper, and I'll more than likely buy another. They are well built, and yes- you can certainly 4 season camp in them.
This was our base camp for 2 weeks. This was the day before the blizzard rolled in. It was 65deg and overcast. All my friends in the outfitter tent bolted until after the storm. I stayed.
Next morning it was -10 and coming in waves. Eventually we got over 2ft of snow and mega drifts.
My camper buddy 'Double Dutch'.
I routinely traveled with that rig in the dead of winter and went ice fishing up around Ft. Peck or around Canyon Ferry. No problems. None. I carried a 100lb propane bottle in the camper on the floor (strapped) and had at it.