interesting find. My brand new Sabre has a similar leak that I haven't been able to track down yet. I thought it was the shower door leaking, so dismantled the frame and bottom rail, cleaned it all up, added lexan, and re-installed everything, finishing with a bead of lexan inside the bottom rail. Still leaks. Like you, I find water in the drivers storage bay, but also the bedroom carpet is damp so it's weeping a ways.
So when you say rear of shower, do you mean a seam? My shower is a residential style, so shouldn't leak.
Joel and Teresa
2016 Sabre 34TBOK
2006 GMC 3500 CC LB DRW 4X4