Let me toss in my favorite, stuck shuttle valve. If slides go in/out then there is power to the control board. Get the system manual and look at LEDs on control board, when power is applied to each solenoid you will see led light. see this link, HWH Repair Sheets
Also solenoid will get hot, too hot to touch,(poor design, IMHO) if problem isn't in the wire harness/ground. The Shuttle valve is only in jack loop. When you try to extend jack you should hear pump going, lower sound volume then when under pressure extending jacks, about like when extending slides. Means relief valve opening, putting ATF back into tank. There is a troubleshooting procedure for solenoid valve/shuttle valve problems http://hwhcorp.com/mi158813.pdf
The shuttle valve design is not robust (IMPO) and is easily jammed by a little piece of foreign material. I think it could be washed out with DEX III and reused if there is no physical damage, but HWH says to replace it if the pin is stuck. Cost is about $160 for a replacement if you need it, and it takes a 7/8 wrench to remove it. Replacement comes with necessary O-rings and backing rings as well as flare fitting at the top. Took me about a hour from start to finish to replace ours, but HWH says it is a 15 minute job
. Only real tricky part is alignment of the two elbows for the U-tube during restoration (see *.pdf referenced above). O-rings are easily damaged and can introduce foreign material into the HWH system when shuttle valve or solenoids are replaced (again IMPO) which can show up long after the replacement job.