If you use the furnace sparingly, you'll be just fine. We like to heat our up before bed, then shut it off, or turn it way down. Then, warm it back up in the morning. What kind of lights do you have. If you've switched to LED, they will have a very minimal draw on the batteries. If they are incandescent, they can drain the batteries pretty quick. Just use as little light as possible. When we're going to be out for several days, we'll just use a battery operated lantern inside the rig most of the time.
2016 Ford F350 Turbo Diesel SRW 4x4
2017 Grand Design Reflection 297RSTS
2013 Ford F350 Turbo Diesel SRW 4x4 (Destroyed by fire - 8/29/16)
2014 Grand Design Reflection 337RLS (Destroyed by fire - 8/29/16)