JOGlesby - for every question you should include your RV make, model and year. Fortunately you included this info on your profile page so I looked up your floor plan (screen shot below). The experts in this forum can offer more help if they know which RV you own. 2021 Keystone Bullet 290BHS.
It definitely sounds like a plumbing leak - not a slide leak.
In the floor plan below - you are only finding moisture at location “D” under the sink?
You stated that you “drained the water heater”…did you also drain the whole plumbing system using the low water point drains?
How long has it been that you have noticed the dampness and soft floor?
If you had drained the plumbing system/water heater and the moisture showed up (say) after a rain, then perhaps it could be a leak from the slide.
'19 Chevy 3500 High Country DRW
'18 Montana 3921FB Aggie Ď77
('17 Laredo 330RL previous RV)
In the sticks near Austin TX
Aussie Gus+Texas Heeler Jimmy