what model TT? see any water marks on walls or is it just at the base of the wall? I'd hose things down and see if you can pinpoint the rough area where the water is entering but like above, start re-sealing all the exterior lights, tire mount 9of on back wall, and any doors etc in that area.
*Currently trailer-less but shopping
07' GMC Envoy w/ P2 controller (current TV)
88' Chevy K5 Blazer (soon-to-be TV)
74' VW Superbeetle (1303)