East Coast Campers around Inch could have a squint at it perhaps.
First i would check all the joints - are there any deformities, rubbers missing - anywhere it has been hit. :blink:
:bash: Check the condition of all rubber seals - joins, windows, roof lights, wheel arches, water run offs.
Check the drainage of the sink, showers, water storage and waste tanks. :bash:
Go to a Property letting agent or house sales place- give them a £5 er and ask to borrow their moisture meter, strip out the unit - totally - everything that is held in by two screws and start prodding the walls, floor areas till you find the extent of the moisture and perhaps the source could be found?
Remember where everything went that you stripped out - photo - phone video etc
That should take a Saturday up and thereafter if no joy
- go get help!