danrclem wrote:
... I doubt if I can buy the sides without buying the pan as a kit. ...
Go to Lowe's or Home Depot and pick up a couple sheets of that soft rubbery wall paneling that's made to resist water. Cut each to size, and attach to the wall with a bunch of Liquid Nails (same as a house). Then use bath tub calking down the seam in the corner.
When I remodeled the bathroom in my house, this is what I used all around the shower - bathtub and extended it beyond around the toilet. Actually, 1/2 of that bathroom has this on the walls. Its been there for over 2 years now, bet's showered on sometimes twice every day, and shows absolutely no signs of wear.
It a LOT, LOT, LOT cheaper than trying to buy a surround specific for your RV. A LOT! And will probably be better too!
EDIT:
I dug through my old photos and found these. I remodeled the bathroom in September 2015. Here's what I'm talking about. I don't know what this material is called, (on the walls), but it sure is a lot cheaper than buying something from an RV supply shop.
(And yes, I did this remodel myself. It all started with the floor rotted out, from ... you guessed it ... a leaky shower pan. Took the shower out completely, removed a closet, opened it up, and gutted the entire room out. Left the tub and toilet in place, everything else moved and replaced. It had glass mirrors surrounding the tub, which had no shower originally).