I think the hose test is a good idea, I would do the same. Than take it back to the dealer and explain everything you did to locate the leak. It could save time at the dealer if you prove to them where and how the water is getting into the trailer. Because it is under warrenty I would have them fix it.
2013 24RKSWE (27ft TT) Cougar 1/2 ton series
2013 Ford F350 4x4 CC 6.7 engine, 8 ft bed 3.55 rear end lariat package
Retired from Oregon State Police in 2011 than worked another 9.5 years as a small town traffic cop:
As of 05-2020, I am all done with 39 years total police work. No more uniforms for me.