I went the expensive route, used fan heater, just a little green house one. It is expensive but circulates the air better around everything, when was down to minus 15+20 even that wasnt good enough.
As a little thing i do too, dont know if helps, but during winter i always drain water out of the brush, the pipes are very thin and in my mind weak in that, last thing you want is that pipe freezing and splitting.
Planning on boarding my van out with thick ply and insulation...that is when can afford it, as heat goes straight out again. Helps if can park under cover too, car port, garage, etc.