Okay after further research it looks like my furnace "sail switch" isn't getting up to enough air flow capacity on battery to trigger the gas main.
I also just learned that my 2008 GMC Sierra has an RVC voltage regulator that lowers the alternator output to like 12.5 - 12.8v to reduce engine load or some such... so my camper battery isn't even getting fully charged! Sigh. Haha. Going to disable that and pump 14v into it like it's meant to get. Probably will solve this entire thing.
Maybe sometimes "talking" to oneself on a forum is good enough for diagnosing things. Haha.
