I installed a Transferflow 50gal (with tool box on top) in the bed of our F250 in Oct '04 (7k miles on the truck) and have had no problems (now 47k miles).
The system monitors the level in the main tank and meters out about 4 gals at a time when necessary. There's a digital meter mounted on the dash which gives me gallons in the main, aux, and total.
We have an awesome road to travel when we go to Punta Bufeo, so I would think that any leaks or trouble spots would show themselves quickly (especially with diesel fuel).
I'm a proponent of auxiliary tanks rather than carrying Jerry cans or other such extra containers. I could have purchased a replacement tank for the OEM tank which would have given us substantially more than the 38-gal std tank, but I opted to lose a bit of storage space for the 88 gal capacity we now have.
With our present mpg, our range is over 1500 miles w/o a re-fill. When we left Prescott a couple of weeks ago my re-fill target was the Flying J T-stop in Twin Falls, ID, where fuel was 20¢/gal cheaper than here. Put 50 gals in (had almost 40 still in the tanks) and bought lunch with the savings. :beer: