Bill, I did this on my Motorhome.
The problem is if you use the extend a stay you will be tapped into the propane system between the on-board propane tank and regulator. So to use the extend a stay you need a direct high pressure hose from only one tank.
What I did was piece together a Duel stage auto changeover regulator

so that I can use 2 - 30lb tanks. I then remove the MH's low pressure (rubber) propane feed and replaced it with the one coming from the auto changeover regulator.. basically removing the on-board propane tank.
So over the winter I use just the 2 30lb tanks.. and then when I'm ready to travel in the spring I remove them and re-attach the onboard tank.