I added 2) 4 gallon rotopax's underneath (where the spare tire used to be) over 2 years ago and haven't had any problems what so ever. I'm lifted, locked, dual cased, blah blah blah. I look for difficult trails that lead to remote places, and it's nice to have the extra fuel. My rig is not a daily driver and I view the rotopax as 'emergency fuel'. It would be nice to have an auxiliary tank or relocate the tank, but I didn't want to go thru the hassle.
By the way, they still swell in the summer (it gets over 100' here), but probably not as bad as if they were on the roof, in direct sunlight. The amount mine have swelled, never prevented me from removing or replacing them, just made it slightly more difficult.
Also, you have to get completely underneath the truck to remove or install them. I don't mind this. I just watch my fuel gauge, and if I know I'm getting low, I find a flat dry spot to 'fuel up'.
Durwin