Yes you can run lights, water pump and furnace off of 12V. But, you will need a generator to keep the battery charged. Even a 1000 watt model will do. Best suggestion would be a pair of 6V golf cart batteries in series. Strongly recommend that you upgrade the Converter to a modern 3 stage type.
'90 GMC R2500 7.4L w/ Gear Vendor OD
'90 Avion 34V
Cummins Onan P4300ie
Pro Pride 3P
Prodigy