You might be able to poke the subfloor by reaching up through the rear bumper storage compartment. Your trailer and mine have OSB subfloor material, not the best for wet conditions. Our trailers are not known for water intrusion at the joint between the rear of the shell and subfloor like the 70s or mid 2000s. Check to see if your subfloor is getting soft back there. You might be able to gain access to the rear subfloor under the beds or furniture at the rear of the trailer. Again, check for a soft floor.
BUT, the rear joint does get a lot of water pressure during a rain storm. You might consider removing the trim piece at the rear, and caulking the rear seam to help avoid water intrusion.