The answer to your problem probably lies with getting the cylinder out of the RV and finding a hydraulic shop that can either handle the rebuild or even build you a new one to spec.
I've heard other RVers say that HWH doesn't have any of these old parts in stock and that they can take 6mo or longer to build a new part. They are more focused on OEM projects - like the new Winnebago Journey motorhome where the slides and levelers are all HWH products. There is probably more money and more production consistency in selling to manufacturers than taking care of individual customers with older products.
Not positive if this video directly applies to your situation, but it might help. This is part one of a three part video where youtuber AZ Expert takes a slide out out of a 2002 Journey and fixes a leaking HWH cylinder in the process.
The HWH part starts approx 6min and 20secs into the video;