I would start simple...Is the fresh water drain valve open?
The fill tube, overflow vent, and drain valve on both our '87 and '06 are on the curb side, but other models might be different.
If your fresh water tank drain valve is closed, then I would think that you may have a more serious problem.
Carl, Elaine & Finn The Beagle
2006 30' Classic W Slide & Limited Package Katarina
2006 GMC 2500 HD 6.6 Turbo Diesel Crew Cab 8' Bed
NQ3U (was KB3UOM)