All of HWH's solenoid valves have a business end that looks like this (note this valve end is missing a two-o-ring pair that goes in the square groove near the large cylinder end; those rings are required to seal the valve against the manifold hole):
Hydraulic fluid pushes in the side holes of these valves inside the manifold when the pump is running. One of the solenoid valves gets +12V to the coil housed in the cylindrical section (off camera above left of this photo) which creates electro-magnetic pull to withdraw a needle out of the hole in the end of this "nose piece." Fluid then exits into the hydraulic hose tapped into the manifold at the end of the valve.
You can barely see the needle in the side hole. Note the hex panel nut just up & left of the side hole. That is the keeper that keeps the nose piece at the correct tension against the needle's conical point to seal against ~2,500psi. This is an old valve w/only 2 o-rings (one round & one square) on the nose piece, current models have 3, 2 square rings + 1 round (standard shape).
You can adjust the tension of the needle against the end orifice inside the nose piece. Remove the offending valve in the usual manner, loosen the panel nut. Remove the nose piece & make sure the inside orifice isn't damaged or clogged w/debris that makes seating the needle impossible; you can clear any debris & reassemble, but if the orifice is damaged that's a machining job w/tolerances probably outside average DIY work.
Reassemble till nose is snug against needle, then turn the nose piece so the needle has good pressure against the orifice. Tighten the panel nut, remembering it is steel & the nose piece threads are aluminum & you can strip them if you go nuts.
Re-install the valve & test. If you need new o-rings you can contact me by PM or HWH. Its easy to clip an o-ring on the extremely sharp-edged internal "lands" of the manifold & cause a leak. There is a wild chance that the 3-o-ring assembly has been compromised & is leaking pressure back into the manifold as well, tho if it was a clean install initially & worked before (till it didn't) the o-rings are probably OK now (i.e. its the needle seat) & the only danger is reassembly & clipping one.
All this assumes the needle seat is the culprit. Could also be the solenoid valve spring that drives the needle against the seat is weakened. If that's the case & its getting bad, you may need a rebuilt valve (gotta ship to HWH) or a new one which they will sell you. If you want to send yours in for rebuild, & need a plug for the hole to continue using the coach in the mean time (manifold will leak all the while you have the valve out), you can PM me for a custom machined plug that will seal the manifold & you can use the coach except for the offending slide(s).
Hope this helps.