First I would read the manual and follow the steps to check the system.
If that doesn't help, I would have him call HWH Factory in Moscow Iowa. They can help troubleshoot. They are so busy right now that it might be 2 weeks before a technician can call him back.
On our coach, parking brake must be on and transmission in neutral. Batteries must be charged.
We just went through some problems with our 2004 Country Coach. Only difference is that we have air bag leveling and not actual jacks. We took ours to the factory in Iowa. There are many components. 2 of our air valves were stuck) and you're right, the technician whacked it to find out it was bad. They also rebuilt the black "brain" box while we were at the factory.