I owned and camped in VW van-based RVs for 20 years before buying a Class A. My last one was a 2000 Winnebago Eurovan Camper, with the pop-top. The Eurovan-based campers, like your Rialta, can be very frustrating, mostly because it is so difficult to find competent service at a reasonable cost.
It will help tremendously if you can gain some understanding of the Volkswagen part of your camper. There are many of them on the road, and an active community of hands-on owners that can steer you toward good parts and service resources. Despite its age, even Winnebago (and Lichtsinn RV, https://www.lichtsinn.com
a dealer close to the factory) have tons of documentation on the systems and parts in your Rialta.
Winnebago purchased cab/chassis units from VW in Germany, and had them shipped to the USA for installation of the body and camping equipment. There are subtle differences in the engine and transmission electronics between these imported unfinished vehicles and non-camper vans that only a VW specialist or dealer would have information on. Bosch electronics do not age well. Many problems are ultimately linked to corroded electrical connections, especially the chassis ground points.
The old VW van community is very active and helpful, but even there, Eurovan (known as the T4 outside of North America) knowledge is less common than for classic rear-engine models. Few people will know or care about the Winnebago parts, but the VW issues are well-known and solvable with a bit of knowledgeable troubleshooting.
I would start reading here:
Here are a couple of parts sources that are far less expensive than the VW dealer network. I can say with no exaggeration that I have spent thousands of dollars on parts from these vendors, and have been satisfied with their pricing and customer service for a long time. I would only use the VW dealer as a last resort for the very few parts that are available nowhere else.
Europarts of San Diego
Please feel free to PM me for more information and references. I still have a full set of the factory service manuals to work from. Good luck with your Rialta. Persistence pays…