Drain cap replacement

Welcome to the forum!
It's really impossible to say without having some idea what year and type of RV you have.
Most of the newer stuff built in the last 20 years is PEX and will just have a simple, one piece cap (no O ring). Pex tubing is Red, blue or white-ish.
Before that there was a product call "Qest". It was all light grey and would have a similar grey cap without any O-ring.
Before that it was usually copper with standard brass fittings.

What type of tubing do you have?

Replace the caps with shut off valves.

Not really sure what lines you are talking about but I will give it a try. If its your low point drains, then I would take a cap to either lowes or home depot and replace them with valves. The plumbing items used in a camper is pretty standard stuff so you should not have a problem.

MANABLOC CAP 3/8"


MANABLOC CAP 1/2"

You most likely need the 3/8"