Check ebay, Amazon, etc. for solar panels and controllers. The furnace draws power for the fan. Be careful about how you use it and you can run for several days without worrying about the battery too much unless it is pretty cold where you are camping. Put in LED for lights and the power consumption for reading, etc. goes way low. My $.02 worth is to try the 45 watt set up. See if it will do the job.
FYI, my approach is to leave the panel mobile so I can set it in the sun when I want to camp in the shade. I intend mounting the panel between the overhang on the camper and the truck cab roof in a mount that allows me to slide the panel out for charging while on the truck or to remove the panel completely so it can be set in the sun if the truck is in the shade. I bought connectors from here:
http://www.solarseller.com/low_voltage_dc_pumps__lvm__teel__accessories__plugs_and_extensions.htm
using D and F from that page and 14 gauge cord to make a 15 foot extension cord. I mounted the 'roof connector' where I could get at it from the front turnbuckle access panel near the battery. I can then plug the cord in and place the panel wherever I want. Until I finish the mount, I will carry the panel loose behind the seat in the truck or in the camper.
Some posts that may be of use:
http://www.wanderthewest.com/forum/index.php?/topic/3303/
and for a roof mount:
http://www.wanderthewest.com/forum/index.php?/topic/2063/