I had two aux tanks (two separate trucks) before selling my 5th wheel.
I went expensive (partly because my read of regulations is different than some and partly because I wanted convenience) and went with American Tank in Frisco, Texas. Their setup is plumbed into the diesel fuel system and into the guage....just flip a switch and you are feeding (and reading) either the main tank or the aux tank.
I went with 95 extra gallons that with the stock tank gave me circa 140 gallons in the truck...effective range even when towing a big-a** 5th wheel of over 1000 miles.
No, I can't pull for 1,000 miles straight. No dog, but the wife can't ride for 1,000 miles either. .......but it's one thing to pull into a lot to stop vs having to pull a 20' truck plus a 40' 5th wheel up to a gas pump. The aux tank allowed me to fuel up in the evening sans trailer....incredibly convenient.