I am not an expert, but here are a few thoughts. My 39k has three slides. In my case the HWH manual says to check the fluid level with the Jacks UP, the bedroom and driver side slides IN, and the passenger slide OUT. That came as a surprise. Doesn't sound right does it.
You might check that the relief valves on the solenoids are all tightly closed. Beyond those items I suspect the shuttle valve.
I suggest a call to HWH. While they are usually a little slow returning the call they are very helpful once you speak to an engineer.