I don't know what unit you have but will attempt to give some thoughts. If you have a metal frame that the curtain rides in, then yes it should be caulked on the OUTSIDE normally with a clear silicone (not the inside). The side rails may be glued or there may be some screws through the rail.
The lav sink drain should be vented, as well as the tub drain and also the grey tank. It sounds like a vent problem. I am guessing but if the grey tank drain valve was closed and you drained a lot of water from the tub it could haveoverfilled the grey tank and forced water up any of the vents. In this instance, I would have thought that the tub would have just stopped draining. The other possibility is that there is no venting on anything (which would really be wrong) and pressurized air from the closed grey tank went up the lav drain and pushed the p-trap water up and out. Interesting situation. Keep us posted.
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