I have to admit... I was seriously looking at this too a couple of years back - and like Rustynuts, came to the same conclusion.
What we have found... The Furnace in the Seneca will amount to 90% of the propane use as we have gone well over 6-months between fill ups in the spring/summer/fall season - but can use up half a tank boondocking in under two weeks in the Winter.
We opted to carry two 20# portable tanks in a storage bay and have an "Extend-a-stay" that can plumb the 20# tanks into the Seneca in the event we get really close to empty. We usually use the 20# tanks for the CampChef Stove too - so they serve dual purpose and as a backup too.
Steve & Stacy,
Oliver, Ned and Ellie ( Yellow Lab, Red Lab, Border Collie/Boxer)
2015 Seneca 36FK
2007 Toyota FJC - on the trailer
2019 Coors Light - in the fridge