What's the best way to terminate plastic water supply pipe?
Jay, it looks like you have a typical RV compression fit PVC line there. Looking at your pics, I don't see any pipe threads on the end of the tubing. It is difficult, if not impossible to match a threaded fitting to this type of pipe. You might have to go to a RV supply with the pipe type(usually printed on the pipe) and purchase an end cap for that specific tubing. If you don't have a pex tool or compression plyers to install a collar, there may be a self sealing compression fitting much like a Shark Bite that can be installed with a wrench, or a simple glue on cap fitting.  Be sure to remove any scored or damaged tubing end before trying to install a new fitting.