We have a 2003 Alpine mid-door 40'and had leaks on both slides...the cause is poor design of the slide tops!...I used a special caulk from WRV to re-seal the full length of the slide top membrane (both inside and outside edges)and the inside corners (the corners are the real problems). I also built small dams (1-1.5"long by 1/4" high) of caulk on the slide tops, at the corners where the outside top edges meet the gaskets on the coach when the slides are fully open. These dams help divert any water collecting on the slide tops to the outside of the coach....Note: saw these dams on the 2006 Apex slides--fact!
2003 40' MDTS
Garden Ridge, Texas