I have plastic surround walls in my shower, they are not tight to the tub pan. I don't think much of the design, but the tub pan does have a lip on it, so it really can't leak unless you're playing water polo in the shower. Those walls would worry me way more than that gap. I could never get comfortable with showering in a wallpapered closet.
My last camper had the same thing, I know it's supposed to be "water proof"
That's nothing a bead of silicone couldn't fix if it worries you. Don't even tell me that you can't use silicone on a camper.