I don't know when RV manufacturers started using
Hepvo Waterless Drain Valves (click here) instead of P-traps, but it's possible your has the Hepvo Valve and not a P-trap.
Here's another link to the same web-site that shows a cut away image of what's going on inside these valves:
Click here.
It's possible, if the drain has not been used for years, it's simply stuck shut. That rubber flapper thing has probably glued itself shut.
You did not say if your camper was previously owned and this is the first time YOU are trying to use it, or if you are the original owner and never used it.
If yours was previously owned, it's possible that rubber thing inside is gummed up with soap and gunk, hair and who knows what else. In other words, it's clogged.
NOW! If yours has an old fashioned P-trap, it's possible the P-trap is simply clogged too. Gunk got into the P-trap and then the water evaporated out of it, leaving a solid blockage now.
Cleaning the P-trap is very easy, but you have to reach it to disconnect it.
Either way, Hepvo Valve or P-trap, you'll need to access under the tub to see what's there, and replace or clean it out.
But, if you are absolutely NOT using the tub, then I wouldn't worry about it at all. Simply dry out the bottom of the tub and call it a day.