Success, by process of elimination.
All four tanks have a bleed valve to bleed all the air out of the tank. So I opened all four tanks. I figure with them all open, the only tank that would have pressure coming out would be the primary.
That would be the small tank on the passenger side of the bus. It had just a little air so my blockage is around this tank. I started by heating the fitting on the input side. I also noticed that the output side tees off to the two larger tanks (one for front breaks and the other for the rear). I still was not getting any more air pressure so I started to heat the tank it self. After a while I started to get water and ice out of the bleed valve. Then I got a lot of water and a lot of pressure. My air dryer over pressure valve stopped venting. I decided to heat the input side until the air coming out of the bleed valve was warm and dry. I closed the bleed valve and got good pressure out of both large tanks. I closed them off and now have pressure for breaks. After they reached 60 psi the small tank on the drivers side started to bleed off. I closed it and now have pressure to my door.
So now I know how my system works.
This in 15 degrees and 2" of snow. We should end up with 4" by morning.