I'm not sure which generator you have but this one looks to run about 5 to 6 hours on a 20 lb tank. Others I've looked at seem to have the about the same runtime.

Propane has a freezing point of -44°F so it should work well at 20°F.

I don't know why you're experiencing the issues you're running into but maybe look into the generator side to see if there's anything that can be checked like the orifice for clogging.

Good luck!