I have used costco and other brands and have not really seen any great difference in performance from any. It all depends on what the right foot is doing. I use Fly J or TA for diesel when using the truck and TT since they have have high turnover and fresh fuel when compared to other places like grocery stores. Will use safeway or fry's when no truck stop around.
Use an additive if you have concerns. You can get the chevron techron at costco for about $15 for 6 maybe less have not bought any lately.
As to going slower with the larger, dont bet on it. We get the same at 65 as we do going slower. Ours is a 2012 Aspect 28ft, but is a B+ so dont have the big wind sail over the cab. Generally getting 8-10 depending on where we are , lower in mountains higher on flats. I do run Mikes 5 Star tune since the v10 is seeming always wanting to go to high rpms and has such poor throttle control. Does make a difference. As we gain mileage on the unit it appears to get better,
