Sounds like you had fun.
The water pump only uses current when it is running, however, you probably have 2 house batteries and a furnace, in 30 degrees, will kill two batteries.
The furnace will draw about 8-12 amps running. Lets say it runs 10 hours over a week-end. (probably more in 30 degrees) that will be 80-120 amp hours. Two 12v house batteries are about 100 each, so just the furnace used 50% of your amp hours available. Now add in any other usage like lights, TV, Frig, etc and you get the idea. Most ckt boards have a low and high voltage cut out on them, so you might be able to burn a light on dim but anything with a ckt board will be dead.
Most folks that dry camp/boondock use a heater like these. They draw no electricity. http://tinyurl.com/ltve3x
I have used a Mr Buddy like this and took the bottle regulator off and connected to the low pressure side of my propane system and it works great. http://tinyurl.com/mktojg
I use the MH9B model. I tied in under the frig, installed a propane quick disconnect, and store the heater when I do not need it inside. DO NOT USE A AIR HOSE DISCONNECT. USE ONLY PROPANE RATED HOSE. A disconnect set can be found here. I used a F276181. http://tinyurl.com/mycy2s
Most of what you need might also be at a good propane distributor, Bass Pro, Camping World.