I would think the construction on a 300x12 is the same as our XLR 27HFS. During this last winter we camped several times with the temp in the low 30's. The heater kept it pleasant inside but it ran almost constantly. Next winter I think we will get a small space heater to help out.
Our tt does not have the enclosed under belly which I am sure would help alot.
John And Mary
2013 XLR 27 HFS
2013 CHEVROLET 2500 6.6 DURAMAX
Nights Camped 2012 (7) 2013 (33) 2014 (27) 2015 (34) 2016 (4)