Based on my own prior experience, and not knowing the specific age of your
Scamp, that looks to me like damage from a leaking window. Check the seals around the
windows. The rear window seal can get dried out and stiff. The putty on the side
windows can lose its grip, and it dries out over time. Also, check the drain holes on the side window. If the trailer sat a long time, it may easily have accumulated a lot of dust and dirt in the "gutter-track", plugging the drains.
Check the rivets for the curtain hooks and any upper cabinets near that corner. They can work loose and leak. Riveting through the insulation and interior fabric, as
Scamp does, doesn't allow the rivet to cinch down properly, so the rivets are VERY prone to movement and leakage. Think of this -- since the curtain hooks are riveted on, through the insulation, and the insulation can compress somewhat when under pressure, that means that if you pull on the curtains, it will pull the curtain rod, thereby pulling down on the curtain hook thereby wiggling the rivets that hold it on. Now... what happens every single time you sit down on the bench seat, catching the curtain between the cushion and your back? Yep -- it pulls down on the curtain.
If that doesn't reveal the leak, look for "spider-cracks" on the radius corner of the roof. It is surprising how much water can seep through the thin fiberglass on these campers when the gel-coat and fiberglass has little stress-fractures in it. If you have a cluster of spider-cracks, get a bottle of
Captain Tolley's Creeping Crack Repair. The stuff works.
Still stumped? I know that a lot of people reject this possibility, but
leaks around the belly-band, where the two fiberglass shells are joined, are common. IN THEORY, the joint is water-tight, but only if the seaming-tape at the joint is 100% sealed to the lower shell. Because of the radius of the curve, small gaps in the seal at the corners are not unusual.
Any spot where water can penetrate the fiberglass shell allows the moisture to get behind the insulation. It then runs down to the floor, finds a penetration (such as the strap-screw for your water tank), and then rots the floor. Since
Scamp coats the underbelly and much of the floor with resin, once water gets into the floor, it has a very hard time getting back out. When the water comes in this way, you may NEVER see a drop of water inside, and never know there is a problem (even looking from underneath) until the floor is well rotted.
It looks like you caught yours before the damage extended too far. Should be a reasonable fix. You'll be done in plenty of time to get camping!