Call the Love's Truck center in Shorter, AL ask for Jamie . He still works there he is an ex army mechanic for 20yrs. He knows 5 tons.
Start with the basics.
Did you push your air dyer blow off valve back in? You know they stick open.
Shut off the primary tank and see if you can build air on the secondary tank. If no ,then close it and see if you can build primary pressure with the secondary tank closed.
If They wont build pressure:
Pull your charge line off the drivers side of the air dryer. see if you have air coming into it. . If so, put it back on. Pull the other side of the dryer off going to the wet tank, see if you have air pressure. If no you have a stuck air dryer one way valve. you need a half inch bushing to hook the lines together bypassing the dryer.
If not I can be there in 6hrs.
Other thing to check is this. Pull your one way valve that is behind your step off. and see if you have air going to the spring brake tank. It can cause low air pressure as well. If you have air coming out of it. The large round Kn23000 valve. If not. Pull the green line off the backside that comes down the frame rail from the rear to it. See if you have air. You should as it is a direct feed from the primary tank.
I put the word out to swamp, and to Robert who may be at his camp near the AL/GA line off I85.