Montreal, You need to adjust the valve arm. Here is a picture of a valve not exactly like yours but similar.
You loosen the nut (see arrow) and move the arm on the valve. Hold the arm base (the white block) and the arm will move. A small adjustment will change the ride height quite a bit so start out with small adjustments. There is also a delay of a minute or two built into the valve.
If you move the actuator rod up and hold it after a couple of minutes the bags will inflate and lift the coach. If you lower the rod after the delay the valve will release air, deflate the bags and lower the coach.
Reattach the actuator rod, operate the compressor to pressurize the system and make small adjustments to the valve and give the valve time to react.
When you are going down the highway the lower cross member moving the actuator rod tells the valve to add air or release air to keep the ride height constant. The delay in the valve saves air by not reacting to every little bump or pot hole in the road.
Good luck, Dan