I added the 14 gallon passenger's side auxiliary tank, details are found in my build thread. See posts 613-616 on page 62, posts 646-649 on page 65, and posts 682-684 & 688 on page 69. http://www.sportsmobileforum.com/for...-16293-62.html
We used to carry an extra 10 gallons of fuel in two containers on top of the swing arm mounted Aluminess box in the back. I could feel that extra weight back there and I didn't care for it. We still carry the containers and they will get used on occasion, but definitely not on a regular basis.
The downside of the passenger side tank is that the fill nozzle is located on the passenger's side, which means that the van needs to be moved when filling both tanks at once. This isn't a big deal unless the station is full and cars are lined up (think Costco), just pick your time and location when filling both tanks.
I originally thought I'd leave the auxiliary tank empty most of the time, but that has not proven to be the case. I can't feel the extra weight when driving and the additional peace of mind is well worth having.
When I transfer fuel I typically do so when driving, thus the time it takes is a non-issue. FYI, the pump is rated to move the fuel at 0.83 gpm.
I suggest you give consideration to the bottom profile of the tank and your use case for the van when deciding which is the right tank for you. A smooth, strong bottom that could be drug over rocks below was a prime consideration in my selection. I've seen the bottom side of the larger (and very expensive) replacement tank and it would not be for me.
All in all, this modification gets a big thumbs up!