I'm not so sure it is all that much cheaper. With TransferFlow and some of the others you are buying a complete system with pumps, electronic monitors, fittings, etc. That is a lot of the cost. Transferflow is a super system, but I wanted simplicity. With my tank I just transfer fuel to my stock tank thru a fitting I put in the filler neck. I used an electric fuel pump, filter, and fuel shut-off solenoid to build my own system. That added about $150 to the total cost. I also wanted the ability to remove my tank easily if I did not need it. I just remove the fuel and vent lines and plug them and remove a couple of bolts and I can lift my tank out (if it is about empty). Takes less than 10 minutes. The pump, filter and solenoid are mounted under the truck bed.
I used an RDS tank on my previous '97 RAM CTD for over 6 years and the tank still looked almost new when I sold the truck. I did not need the toolbox combo so I got just the tank. The one on my 2004. 5 is only extends 9" from the front to back and is about 1" taller than the top of the box. That helps in my shortbed. I use mine mostly when towing my 5th wheel and the 45 extra gallons lets me go all day without having to make a fuel stop. My tank has fuel and vent fittings, a fuel gauge sender plate, and a 2" threaded opening with plug (for an external transfer pump)on top and a drain fitting at the bottom of the tank. Even though I ordered mine thru Northern I called RDS directly to ask questions about the tank. They were very helpful and were glad to answer all my questions.