Look up! :B
Do you have ducted air conditioning?
If so, Is this vent right above that water spot?
I have only experienced 'this' SINCE being in Florida's high humidity.
AND the humidity quadruples after it rains.
If you have the air on and keep going in and out of the RV,or leave the door open longer than you should when you do, or have left a window cracked....HEAVY condensation will form around the a/c vents.
You don't leave that roof vent over the bed open when the ac is on do you??
I had the same mystery water spot and I finally found out it was the condensation dripping from around the ac vent in the ceiling. This is now my 'warning' sign when I have forgotten to close a window after turning the air on. :(