My 2013 990 was regularly stored for 9-months just on the jacks with 2/3 full tanks before I bought it used a year ago. There was/is no sagging or other issue. Before I bought it I spoke with one of the AF guys who supposedly has some design input into them and he said the same thing about filling the tank with a positive seal ballooning the tank and breaking the supports.
I mostly store it on the truck now, but it does sit unsupported for a few weeks at a time when I need the truck for other stuff. Adding extra support seems like overkill to me, but time will tell.