Hi Ho: I'm beginning to think this problem is endemic, but yours is a little different from mine. You can (should) check the T valves that are located on the pump under the hood. Just open the hood and look down. You can check the fluid level while you are at it. ATF is ok; the HWH fluid is just ATF without the red color. Make sure the T valves are tight. Since the jack won't pump down there are two possibilities: Either the valve at the pump is not providing fluid or the pump itself is stuck. It is pretty easy to interchange the the front and rear lines at the pump. Just make sure you put a rag over the fitting in case there is still pressure there. That will tell you if the jack itself works or if the problem is up front. Calling HWH is also possible; they are pretty helpful. Your users book is also pretty good and will tell you which valve controls which jack etc.
I had the problem 2 years ago of one jack not extending with the other three. HWH ask me do some checks on the main controller box while on the phone. They sent me a new controller box, aluminum box just under steering column of 2001 Itasca. I installed per their instructions, took 10 minutes. All jacks have worked perfectly since. This box is apparently the brain for the whole system.
Went to HWH website. Their troubleshooting guide was better than my manual. Also emailed for help. They responded today. Turns out the RR fuse in control modual popped.I replaced it and it is working fine. Rep said as long as it don't keep popping the fuse I should be ok.
Don the control module is valcroed to the fire wall just above the steering column. It has a black valcro strap around it and has two plugs, one at each end of the box. The box is aluminum color with dimensions of approx 1 1/2" deep, 5" X 7" with cooling fins molded into one side as I remember. I haven't looked at it since I replaced it in 2002.