I have the RDS 40 gallon tank/toolbox combo, giving me 78 gallons in total. I think RDS gives you the best bang for your buck. Yes, the hose that runs to the OEM fuel line is small but I don 't care. I keep the valve closed while driving and then flip it open when I stop for the night, then flip closed again in the morning. It refills the OEM tank overnight.
Also, like others have stated, I go fill up after I have set up at the campground. Much easier and I usually combine the trip with a dinner or grocery run. The extra tank allows me to buy fuel when I want to, not when I have to.
If I had it to do over, I would get the 60 gallon tank/toolbox combo. The footprint is exactly the same, but with a smaller toolbox.