On the backside of water heater is where valving is located (inside RV)
Sounds like check valve on hot outlet is plugged/failing.
The check valve is inside the nipple that is screwed into tank (top line)
They are cheap plastic plunger/spring/o-ring assembly.....plugged due to scale/debris or o-ring is out of position.
Check that you have water/getting in WH tank.
Turn off water supply
Pop open the PRV via lever (outside compartment)
Water should drain out if tank is filling (eliminates problem with cold inlet valve being the problem) You can turn on water supply with PRV open to check that water is flowing into tank.
If it is...then check valve bad. They are junk to begin with.
Some folks just remove internals and install a shut-off valve in hot outlet line
Check valve is just to stop back flow into tank when bypassing water heater
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31