There are two ways to do this:
Get an Extend a Stay kit or buy the equivalent fittings at your hardware store and splice it into the high pressure propane line in between the propane tank's shut off valve and the RV's regulator. Then hook up a propane tank directly with a high pressure hose, close the RV's propane tank valve and then ue a 20# tank in place of the permanent tank.
The other way and the one I prefer if you have an external BBQ propane fitting is to hook up to that fitting, which is usually below the bumper. Hook up with a male quick connect fitting, an LP hose (low pressure ok) and a high volume two stage regulator at your 20# tank. Plug it in, close the RV tank valve and open the valve on the 20# tank and it starts supplying your RV.
The disadvantage of this scheme is if the quck connect fitting becomes disconnected, propane will spew out of the 20# tank's hose, so you have to be careful. But it is easy and doesn't require figuring out how to find room for the extend a stay fittings.
David
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