[Mtntrek]

Sorry to hear you're having problems, I'm sure it's frustrating to deal with.
Your initial landing gear problems sound all too familiar, or indicative of a internal seal problem generally associated with the original hydraulic cylinders. Random Sudden.
With these hydraulic systems, one common pump, actuating multiple devices. Any seal, valve seat, check valve failure is likely going to effect something else, possible slide or leveling jack interactions.
In regard to the finger tight, loose fitting. I would expect it to be torqued with a wrench as well as possibly having thread sealant. Sloppy trouble shooting somewhere?
The control valve coil stem probably has an O-ring where it attaches to the valve body. Which would also be torqued appropriately.
I have no experience with the PC board, but would suspect it's purpose was to sequence slide operation. Or PREVENT multiple slides from operating simultaneously. Perhaps there was a remote control function on some models? It would seem the LCI - Level Up folks might have a clue? Schematic for a PC card might be a stretch, with any mfr. though. Puzzling that DRV might express ignorance, but it does seem that any RV "components" can be here today and gone tomorrow.
Perhaps others have experience with the option.