If you have located the water tank, I'd crawl under and look for a drain from there. The drain might have a rubber flap on it to prevent insects and dirt from getting in the opening. Once you find the drain, go into the area above it and look for a valve.
On your question about the switch for electric dump valves, I would do a through search of every cabinet and closet. Who knows what you might find? Doing a complete survey of all spaces is a good idea, especially on a used, 'customized' RV. Writing down any labels, manufacturer and model number of objects can allow you to search on line and find any manuals for systems and appliances you discover.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI