Welcome aboard!
28 degree at night and 50 during the day is nothing to worry about . We do camp in much cooler weather than this most years. We do not have your tt model. Out htt does not have an enclosed underbelly. Coolest we have been is the low 20's. We do not have any tank heater. No issues.
Fill your FW tank full. The thermal mass will keep it from freezing. Hose, disconnect, drain, store in a cargo hold at night. If you leave it outside it will be frozen, and super stiff in the morning. If you have sewer connection, disconnect this too, or it will be full of frozen condensate.
This time of year, we tend to primarily use electric heat. So we just keep the cabinet doors ajar that have plumbing run through them. This allows a littmore heat back there.
Have fun.