The Extend-a-Stay is no longer made however Camco has an identical device as does Mr. heater (Stay-A-While)
These come with an adapter that goes between the tank valve on your RV and the regulator.. Turn off Propane, remove POL (left hand threaded) nut from valve and push aside, Screw Adapter into valve, screw original fitting into adapter. You may need to add some support for the regulator (or not).
That is the install
Adapter comes by it self or with hoses.. The one you want comes with two hoses, one is a 10 foot adapter to grill hose, this has an end that looks like the top of a Disposable Propane canister (those 1 pound or there about jobs) The other end is the mating connector. (looks like the one on the grill).
The other hose is 5' and connects to a portable tank and then to the adapter (it has a special inlet fitting on it)
If you are using a bigger tank, the tank supplier can likely deal with the extensions you need.
They work very well, I have one. I have seen many "Schemes" that scare me, this one I approve of.