[Fulltiming]

1st, it tells me that since the issue does NOT appear to occur in the travel mode, so it is likely NOT the ride height valve or the travel mode solenoid.

I would focus on either a stuck dump solenoid for the left rear (could be the left rear solenoid has debris in it preventing it from closing you should hear air releasing all the time the coach is in level mode if the dump solenoid is stuck open) OR a blockage in the six pack to the left rear raise solenoid or in the line from the six pack to the left rear airbag.

I presume you have tried putting the coach in travel mode and letting the bags all air up and then switching it back to level mode and exercising the dump valve for the left rear.