I actually just asked a friend of mine that works in the service department at the Jayco factory. I have the same issue. He says not to seal it up. The gap behind the tub surround has to breath or you will have mold buildup behind it. The tub itself has a 2 inch lip that runs up the wall behind the surround. You shouldn't have any problems with shower water getting in there. The same wall material through out your TT is what is behind the surround.
2015 Jayco 24FBS Elite Jay Flight
2013 Chevrolet Silverado 2500HD 4X4 6.0L 3.73 Axle
HD Trailering Package with Integrated Brake Controller
Equal-i-zer Hitch 10k/1000