Since the dash gauges are all run these days on a CAN bus
, and since you've already inspected and cleaned the grounds, it's time to go under the RV and check those giant connectors that interconnect all the vehicle computers to the dash. And modern vehicles have lots of computers. They all communicate with the dash via the CAN bus.
If you have an intermittent group of readouts on the dash, it's very likely a corroded or damaged connector. So start your search where it's likely that road water splashed up onto a connector. Look for the nastiest connector and clean that first. You may need to find the MoaB (Mother of all Boxes) under the chassis where all the connectors come together. As that is often the CAN controller that interfaces all the incoming signals. Remove, spray clean, and re-seat every connector there. And all of them in the bundles of wires going way back to the rear of the RV too.
Note there are big connectors on most engines and transmissions these days too.
I know, it sounds like a lot of work, but would you rather pay a shop to find it? Or do it yourself. With a bit of brain work, shouldn't take too long.