The OP was having trouble with the furnace when camped at Mammoth Lakes. What is the altitude there? It could be that high altitude is making it harder for the furnace fan to get enough flow. Combine that with a marginal battery charge...
On a trip in the Rockies last fall, I had an erratic furnace when camped at 7800 feet in Yellowstone. The rest of the trip it was fine.
I also have a Rockwood MiniLite, with a WFCO internal charger. As others have mentioned, it charges quite slowly. When I am recharging from a generator, I use an external charger (B&D 1093DBD) I have to connect it up each time, but I've avoided the work and expense of changing the internal converter. When I am on shore power I let the WFCO do the charging. It's slow. but it's gentler on the battery, and I do have all night.