I just had the same thing happen on my '86 Sovereign
. It was the plug on the bottom of the potable water tank. There are some threads on this problem (I'm not sure how to put a link here). The hardest thing for me was to remove the cover for access. The plug has a hex type hole in it. A 1/2" bolt head fits right in the hole so you can turn the plug out. Put two nuts locked on each other so you can turn the bolt. Turn out the plug gently. Clean it up & put either teflon tape or plumber's putty on it (I used the putty). It should be fine for another 20 years or so. Some people replaced their plugs, but mine was in good shape. Make sure yours hasn't shrunk before you put it back in. I put my cover back on with rivets because the screws were really hard to get out.