I could see some daylight under the slide on my 2503S. I've caulked the heck out of it on both sides. There's a "fender-shaped" piece of plastic above the wheels. You need to caulk where it meets the side wall of the TT. It's kind of hard to get into it in some places. I had the DW pull the decorative piece away as far as possible. In tight areas, I put the caulk on my finger and put it up where it needed to go. (Be careful not to cut you finger on the plastic - don't ask me how I know.)
I think to really do it right, you'd need to remove the decorative piece. It appears to just screw to the side wall; but I haven't felt the need to go that far, yet.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10