One of my tank guages wont build past 25 psi. (One on the left builds to 120.psi) I just replaced a pressure protection valve that was leaking bad and thought that would fix my low psi reading but it didn't.
After the air system builds to full pressure and I shut down engine I don't hear any bad leaks. Could this be a bad air sending unit on the tank?
Your coach has the infamous blue or some other color box that goes wonky.
Air Pressure won't build (https://www.foreforums.com/index.php?topic=45617.msg463702#msg463702)
With only 25psi in the tank, the beeper in the dash should still be going off for low air pressure. Is it?
Yes, and the red light is on.
If you have a threaded water drain valve on both tanks, you should be able to connect a small inexpensive pressure gauge to the tank and read the actual tank pressure.
If it agrees with your dash gauge, you have a mechanical problem. Either air pressure isn't reaching the tank, or it is leaking out.
if your dash gauge is lying to you, then it's a sensor problem. See craneman's post.
My "blue box" hasn't entirely failed but my dash gauges are notoriously inaccurate and I have been through two of the pressure senders on the air tanks (which are ridiculously overpriced and only available at Summit Racing as near as I can tell). I like redundancy so I installed two VDO mechanical air pressure gauges in the dash in addition to the OEM electronic pressure gauges. Gives me peace of mind and way more accurate data. For the same reason I also run the Scan Gauge D to redundantly monitor the other gauge functions.
Keep in mind that there is a regulator between your rear and front air tanks that could fail. If I recall correctly, the airline on your coach comes off the front tank, that could be an easy way to determine if air is getting into the front tank or not