Does fuel weight of carrying 2 tanks affect mpg much on IDIs? With my motorhome, I want to stretch every penny I can on fuel. My fuel tank selector switch seems to be working, so I'm debating if I want to use both tanks so I can fill up at the cheapest stations that may be many hundreds of miles apart. The front tank is probably an 18 gallon by how much it takes at the 1/2 tank mark, so at let's say 10mpg, I'd have to refuel after about 120 miles, even if prices are higher than they are another 100 miles down the road. If I have my second tank full, I can keep switch tanks and wait until the next cheapest station. I use GasBuddy so I try to chase down the lowest fuel prices that way, not by luck. But, is it cost effective mpg wise? If it's just a few pennies difference in price, it probably doesn't make sense. But sometimes it's 10-15 cents difference. For 10-12 gallons, that's $1-$2.25 difference. It adds up fast. Especially if I'm about to leave one state where it's 25 cents cheaper than the next.