Hello Gayle and welcome to the forum. I'm not familiar with a Serrano but the drain valve will be near your fresh water tank. On mine it's located on the outside passenger side. It's black in color and resembles a radiator petcock. If you don't see anything that looks like what I described, it might be on the underneath side just under the fresh water tank. Good luck.
Rudy, Jenny and Garrett
2014 Four Winds 35SK Super C - Ford F-550, 6.7 Power Stroke- Blue Ox tow bar with Patriot brake - 2004 CR-V, 2013 F-150 and 2016 Wrangler Sahara Toads - TST 507 tpms