Larry, have you tried Flying J or any other large truck stop? I pick up my fuel additive at Flying J. In general, I have found their truck stop to be very user friendly. They have a separate island for RVs. That is handy when towing.
General opinion I got from the forum is that we should NOT use an additive containing alcohol. The additive I use is non-alcohol and I add one oz. per eight gallons of fuel at each fill-up.
I have no information on the Shell Rotella DFA.
2002; GMC; 2500HD; D/A; extended cab; long box; 4 WD; Summit White; cloth buckets; ONKI running boards, Sierra mud flaps, A.R.E. Z-Series topper, Bed Rug, Fumoto Oil Drain Valve, and McKesh towing mirrors.