30F? is that at sunrise or at 2pm?
You mentioned backflush did not work well... seems like you may have a solid object in the tank. If it really is ice it should clear for doing the back flush a few times. May need a plumber with a camera to scope this out. Maybe Harbor Freight has a low cost camera to buy.
Do you rely on the tank sensors to know the tank is empty? Very common to have a false reading.
I doubt tank and pipe is frozen solid but here is a heat pad you can order:
www.ultraheat.comMay need someone there with experience to actually look at it. Mobile RV tech should be able to set you on the right path in 30 minutes or less.
