I put a 106 gal 100 usable aux tank in our diesel truck I'd purchased from Quality Farm & Fleet (now called Tractor Supply) for $297+ tx but the aux tank's not hooked up to the truck's fuel tank at all. I have a lockable hand pump (one quart delivered with each motion of the handle) (20 motions up and down is 10 gallons) is in the driver's side bung opening and a short real fuel hose to reach the truck's fill neck. The hose has a water line white PVC ball valve on the end and about a 6" long 1" dia piece of PVC glued in the valve for a nozzle. Has a simple PVC not glued on cap to keep the dirt from getting in the nozzle when driving etc. No mess! Works super and saved us a ton of money in the 9 years we've had it. One truck tank of fuel lasts an entire day + of driving so what's the big deal of 10 minutes for refilling it each day from the aux tank? Buy fuel where it's cheap and we have 134 gallons and around 1000 miles range before refilling. I can only use it when we pull a trailer etc and it comes out when the Lance TC goes on. No problem!
I made a welded cradle out of 1" X 1" X 1/8 wall square steel tube picked up on the end corners with 2 made up chains with S hooks that supports the middle of the tank also when I take the take the full or whatever's in it out with the hoist in my building/toy barn behind our home. Four 3/8 SS bolts thru the tank feet from the top secure it in and so easy to spin them out with the SS nut welded to a 1/4" thick steel plate flathead screwed to the underside of the bed from the top. My 5th wheel hitch rails are mounted the same way with four 5/8 grade 8 hex bolts from the top into the 4 frame mounted bolted on "L" brackets. So quick! Takes about 10 minutes to put the hitch or tank in or take it out. When out, both sit on their 2 X 4 frames with 4 HD steel swivel casters under them so I can push it where it want them on the concrete floor.
Didn't put the tank in for this trip and we are sorry now. Wanted more room for my wife's electric scooter but next time, the scooter goes on the cargo rack that's on the back of the 5th wheel and the aux tank will be on the truck! Seen $.53/gal difference in diesel price between home and here in Florida so far. That's $71. for one fillup alone and we'll need a min of 3 fillups or over $200. wasted on one trip! Rolling resistance from 700 lbs extra weight isn't squat. It's the air resistance as it's roughly 3 times the rolling resistance.
Edited to fix a couple typos.