The person who did our orientation suggested rigging up some straps & putting a spare tank in the rear storage compartment - not connected, but just one to swap out. Not sure how safe that would be. Also, I can't afford more weight back there because of problems with swaying.
We have a 2015 6504 & There is a space behind the current propane compartment, quite a bit, actually.
I wonder if your current compartment could be expanded to join up a two tank system? You would still be putting more weight in the rear of the trailer - this has been a real issue for us.
FWIW, we've been out on the road for almost a month & while we've topped off our tank two times, we've not come close to running out. I cook almost every day, use the propane hot water heater switch, used the heater, and we are also are driving 4-5 hours every three days or so with the fridge on propane & we've been cool. We've boondocked a total of 5 days. I thought for sure we'd need another tank, but it doesn't seem like it.
Traveling 9/1/2014 - 11/1/2014
2015 Vibe 6504 • 2005 BMW X5