Just a update. The cap is Not a cap. It's a screw in fastener. Found this out after multiple attempts at pulling it off . The threads had a reddish-brown material in them. No metal screw present. Swabbed the hole with a Q-tip and got nothing back. Am now seeking a replacement fastener
. I truly wonder if F.R. used some kind of adhesive along with the plastic screw- in fastener to fasten the shower surround to the RV walls.