Steve, as Tom indicated, the manual system that you have is not particularly complex. Since the system is working in the 'level' mode, we just need to focus on the 'travel' mode. Travel mode is controlled by the three ride height valves that Tom described. There are two travel mode solenoids (pressurize and dump) in the front (typically behind the driver's side headlights) and two travel mode solenoids (pressurize and dump) for the rear and, IF APPLICABLE, two more for the tag axle (typically in the engine bay on the passenger's side on the wall closest to the front).
I would troubleshoot the system by first raising the coach in level mode all the way at all four corners (you will need the engine running on high idle). I prefer to level by using two switches at a time, either the rear two, the front two, the right two or the left two. That will avoid putting additional stress on the chassis. When the coach is up all the way, then switch to travel mode and see if the coach begins to drop back down to an appropriate travel height that is level with the ground on all four corners. If so, your travel mode dump solenoids are working properly. If not, you may have an inoperative solenoid or a bad ride height valve. Now, put the controls back in level mode and drop just the front. Switch back to travel mode, engine running on high idle, and see if the front comes back up to the correct travel level. If it will not raise at all, you most likely have a bad travel raise solenoid in front although a bad front ride height valve could be responsible.
Of course there could be no voltage going from the switch to the solenoids but that is MUCH more rare than a dysfunctional solenoid. If the solenoids are all working, then the one or more of the actually travel height valves are likely defective or WAY out of adjustment.
If it comes up but not the correct distance, you need to adjust the arm on the front travel height control lever located just behind the center of the front axle. Moving the horizontal arm up on the vertical arm will raise the front of the coach and moving it down will lower the front of the coach. DO NOT MAKE THESE ADJUSTMENTS WITHOUT PUTTING CRIBBING UNDER THE COACH AND PUTTING THE LEVELING SYSTEM BACK IN LEVEL MODE. Getting crushed under the coach as it starts lowering will ruin your entire day.
If the front is returning to level mode properly, then you repeat the procedure on the rear. If after being raised in level mode then being switched to travel mode, both only one side drops then you know which rear travel dump solenoid is bad. If they both drop but one side will not come up, then you know you have either a bad travel up solenoid on that side OR the travel height adjust is messed up on the low side. These are adjusted by moving the horizontal level up or down on the vertical shaft. The right travel height valve is inside of the right rear dually and the left travel height valve is inside of the left rear dually.
If I have confused you, let me know and I will try again. Let us know what you find.
The system pressure to the suspension is typically not adjustable and reads on the gauge in the engine compartment. It should be around 100-110 psi. On coaches with a tag, there is a pressure adjustment and gauge, again in the engine compartment near the solenoids and it should typically be around 25 psi. On coaches with an air shifter for the transmission, there will be still another pressure gauge and adjustment valve and according to Newell it should be set around 60 psi although a tech for the air shifter company told me that he typically sets them at 100 psi.
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