Our '99 has the drain valve located at the upper rear of the water/electric compartment, way in the back of the compartment.
There is a small shelf up there where I keep hose nozzles, etc. Reach all the way to the back and you should feel a 90į turn handle for a valve. When the handle is in line with the hose, it is on. Crossways, it is closed. YMMV
'99 Trop-i-cal 6370 37' / Ford V-10
'92 GMC Sonoma 4.3L 4WD toad