Hi,
In the coldest part of the winter (-37 C, -35 F) I was using 5400 watts of electric heat to keep my RV warm. I kept the propane furnace as a back up heating source.
So, yes you can keep it 70 (21 C). It may require modifications of the "stock" RV to do so.
See the thread under Full Time called Winter RV Camping
In the summer time Air conditioning is going to require shore power, lots of generator run time, or a quite massive solar installation. I think only the first two would apply to a truck camper.