Well it sound like you know what your looking for and have past knowledge of DP so your way ahead of what I know.
I would hope for more like 10 - 12 MPG on a DP but I guess it all depends on your avg speed and grade.
IMO the quaility will be simular to any other Thor products ... so after you carfully inspect everything inside an out (spend 2 + hours doing so) and if you get them down around 35 - 40% off of list..... pull the trigger. Life is too short to be sitting on the sidelines.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor