If you keep the temperature and volume constant you should also have a fixed mass of air, so it might seem strange to think of it, but we are still measuring the relative humidity (which is why we give it as a percentage, and that value is still relative to the maximum amount it could take at that temperature). If it was absolute (the actual water content), we would report it in different units - mass (or mols) of water per mass/mols of air.
...not that it makes any difference to the tub, incubation methods, etc.
You have very nicely shown how difficult humidity is to understand, and I have been trying for three or four years now to get my head around it, with only limited success.