You might want to try this product:
Red Devil 0956 - King Kaulk - White - 9.8 -ounce cartridge at Menards
It comes in black, white, and clear.
It's both a caulk and an adhesive. We used it for the same purpose in the shower on our 2001 Winnebago Adventurer. I was recaulking the seam every 2 years until I found this product. It lasted 5 years and was still holding fine when we traded it off.
Be careful about using a silicone product. Be sure to read your owners manual thoroughly before attempting to use it. Our owners manual said NOT to use silicone because it reacts with the plastic the shower is made out of. I don't know what the end result would have been since I heeded the warning and used a polymer product.