Reply from (NoJeepJarrod)
No sag issues! Replacing the rear coil springs was no more than a half day job. That brought the rear up about 1 3/4". The FJ measured 17 1/4" from the garage floor to the lower edge of the rear tow point before the rear coil spring replacement. Afterward it measured at 19 1/4" and settled after a few months near 1 3/4" change. (Those measurements were with the original BFG tires) As for sag, it's pretty solid, it comes down a little with the trailer hooked up. I'm guessing here, maybe 1 1/4" and thats with full fuel and stuff in the rear cargo area of the FJ. I'm very happy with the suspension changes I went with. Much cheaper than anything else.
As for an additional fuel tank on the FJ, they are way over priced. I know of about two places that make them.
FJ CRUISER AUXILIARY TANK
:: The Long Ranger :: Toyota FJ Cruiser
A few people have settled with a marine style tank mounted on the roof rack. IMO thats just a short term band-aid.
I'd think if you were not off roading the FJ than air bags may be a good way to go. I put my Viair compressor under the hood on the passenger finder with my winch solenoid box.
Well I'm guessing the 16' behind the FJ will pull pretty nice. Whats the empty weight of it? We went with the 22', knowing one of these days we will have a different Toyota to tow with.