Hi Phillip
The heater I have in my van is to gard against frost.
I baught it from Argos its a Glen fan heater they are about £15, they have a frost protection setting and kick in if the temp goes down.
They are very good and I have not noticed any difference in my bills.
But you have to run out an electric lead.
I think its money well spent, if its stops your R/O from freazing as mine cost £600
Roy