I wanted to ask as I couldn't find any info if anyone has retrofitted a larger or auxiliary fuel tank to their X5.

The reason I ask is I tow often with my X5M. With its 22.5 gallon tank when your in rural western areas you have to stop every town which are spread about 80 miles apart (incidentally that was how far trains could go between water refills). Mainly cause running 160 miles between fill ups is pushing the car to fumes. I'd like a 30-40 gallon tank if possible. Yes the X5M with trailer is getting about 8-10mpg.