Adding an accumulator tank

A few OP's here have done this project and should be able to jump in and help you. The story from Lowe's about odd tubing in RV doesn't square with the remark from Lowe's. I wanted a shutoff valve for the supply to our RV toilet and Lowe's had some great choices in sharkbite type fittings made of plastic. I wanted to add a tank drain to our boat's water system and Home Depot had what I needed in metal. My brother phoned on a camping trip needing a fitting for a broken low point drain on his trailer. Stopped at CW first, they did NOT have anything that would help. Next stop was HD and they had a good choice in metal sharkbite type fittings.
Let's say you want to cut a line and put a tee into it, then connect your 3/4" NPT pressure tank. It could require a couple extra fittings to do the adapting.