I plan to install black steel pipe. I have run miles of it in already lived in houses. Back when I got out of contracting, the flexible stainless piping was starting to get popular. I don't believe it is listed for use in RV's, but I could be wrong. The problem however is with the brand I used, you could bend it maybe two or three times and it would break. It is pretty hard stuff, so only good for so many bending cycles. With my black steel lines, I will install a valve next to each unit and use a stainless flexible connector. Those connectors do not seem to have the issues with limited bending cycles. I definitely would not use the brass (usually yellow coated) flex connectors. They are very soft. Connectors are designed for taking on and off the appliance as it is being serviced, so they are made for that. Flexible piping is made for ease of installation, not being regularly disconnected and being bent around.