Depending on your furnace draw, you may need more solar to keep the batteries up. The combination of winter sun angle & low temperatures is going to eat a lot of propane & amp/hrs running the furnace. While a catalytic heater might help with the batteries, it may cause moisture & ventilation problems, and will not provide heat to the tanks, something that some furnaces do.
If you are in one place long enough, you might look into using portable solar panels if you don't have the roof space, as well as an "Extended Stay Propane System" so you don't constantly need to refill the small tank in the RV.
No suggestions for dealing with emptying tanks, other than using a tote if you have another vehicle you can use to haul it to the nearest dump station.