The water is collecting under the left seat of the tounge end of the trailer. You bring up some items I did not think would be a problem.
1. There is an air conditioner installed in the window above the tounge.
2. There is a roof vent.
3. There is inside paneling.
4. There are no stand pipes, maybe?
I will need to go back and look at the trailer with checklist in hand.
Well,Thats a start!
Of course I am not sure which "Left Side" you mean?
Is it the Door Side or the Kitchen Side? The bench on the Kitchen side has all of the Mechanicals under it and may well have water going through there by design. It is easier to picture a problem on that side as there is no water plumbed through the door side,usually.
Mine is also a Paneled interior.It is easy to imagine that water might start at one point and then travel undetected until popping out somewhere else.
I would certainly be suspicous of the Window AC unit.They of course create a steady stream of water when in use and the drain path for that water needs to be looked at.
Every opening inthe roof can be an issue as well.
Last but not least,I have seem Fiberstream's with window ventshades(mine for instance) where there are through-holes in the walls to attach them.
I have also seen 'Streams with screws or bolts through the wall to help hold up the overhead cabinets.
Every place something goes through the Fiberglass is a possible leak point.
The best way to try to find them is to slowly run a hose around where you suspect the problem is until you see water coming in or streaming and try to walk it back to the start.
If you lived around here you could just wait until night time as we seem to have rain every day right now.
Anyway I hope this is a start for you.
can be very tricky.