I fulltime and here's my logic -
Because I full time I am always traveling to new places. Don't know the road, may not know where fuel is exactly, and may never have been to the CG. And I hate surprises !
My F350 has 34.5 gallons in the OEM tank. I bought the RDS 40 gallon tank/toolbox combo, so I carry 74.5 gallons total. I get between 10 and 11 mpg. Let's use 10 for this discussion.
Using 345 miles as my max range on the OEM tank, I would be looking for fuel NLT about the 245 mile mark, giving 100 miles (10 gallons) reserve. Running at 60 mph, that's at about the 4 hour mark. I simply don't want to start searching for fuel that early. And it might mean refueling twice in one day. Not a big deal admittedly, but I want to refuel on my terms, thank you very much.
With the RDS tank I now have a 745 mile range. I don't refuel until the end of the day after I get set up in an RV park or CG . I then take a look at Gasbuddy and look for the cheapest fuel that is near me. Many times, I am heading out for dinner or to a grocery store anyway so it is not a wasted trip.
The aux tank gives me peace of mind. I don't even THINK about fuel during the day. Many times I run the OEM tank below the 10 gallon mark, if I know I'm just a relatively short distance away from my CG. No big deal because I can always stop and turn the RDS tank valve to 'on' if I start getting too close on fuel.
I also get to buy fuel at the cheap places like Racetrack or Murphy (usually close to a Walmart so I can do shopping). Win-Win. The savings will never pay for the tank, But it's nice to save money AND get peace of mind at the same time.
Bathroom breaks are an entirely different subject. There are rest areas along all the interstates, and many places to stop along the 2-lane highways. OR - just stop and use your camper ! It is simply not a big deal.
Been full timing for 3 years now and all I can say is that I'm happy doing it this way. YMMV.
