These trucks are still going through regenerations....As stated, they are far more passive with the DEF addition...I guarantee you that your trucks have gone through regens with that many miles on it...The MPG doesn't take the hit like it did pre-DEF, and they don't last as long..It's harder to "tell" now when you have one in these new-newer trucks...but believe me, if you do the research, you'll see that your truck IS having regen's...Ain't technology grand:B
As far as the the DEF gage, it's non-linear and really not much help...It'll set on full till the 12th of NEVER and start dropping...Lowest mine has gotten is 1/2 tank and I refilled "just in case". I've filled at the same truck stop with same pump and pump until it kicks off..
I've gotten as high as 1600 mpg to DEF and as low as 550-600 mpg of DEF (when towing)...I do all my towing in the mountains. Tow at speeds up to 65 mph.
I'd say OP didn't have full DEF tank when he picked up his truck....
You need to up date your profile with a pic of your new truck:)......What gearing do you have in your new Ram?