There's an "Extend-A-Stay" and an "Extend-A-Flow" and a combo version. The Extend-A-Stay version is for what you're looking for, ability to run propane into your coach's on-board propane system from an auxiliary tank. The Extend-A-Flow part allows for you to run an auxiliary high pressure line off your on-board propane system for BBQs, fire pits, etc.
I did the combo version and I have to say it is one of my favorite mods on my coach. I've used the Extend-A-Flow side every trip, but never had to utilize the Extend-A-Stay part (yet). As A32Duece said, super easy to do, so long as you have room to fit the Extend-A-Stay "T" between your tank and regulator - I had to swap out the short pipe to the right of the regulator for an even shorter pipe so my regulator wouldn't bang against the side wall of the compartment (See before and after pics below). One other note, remember the thread between on the tank side of the pipe between the tank and regulator is a left hand thread - just to save you some frustration (don't ask how I know
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Enjoy!