we have a 2012 winnebago aspect 28T. We tow a 2013 Hyundai Elantra (2700 lbs +-). We just returned froma 4500 mile trip with the rig, our 1st with it. I installed a 5 star tuner prior to the trip and it must work. We drove 62 mph 2300 rpm got around 8.5 mpg, went to 65 mph 2500 rpm got 8 mpg overall. Drove across Iowa to Neb at 65 for 9.3 (hand calc'd). Drove to colorado at 70 mph got 8.3 (hand calc'd). Using the scan gauge as illustrated above had highs at 10 low at 7. All due to terrain and winds. We drive with cruise control on and dash AC. So from what others are saying about no dash ac and slow speed there is not an apparent need to drive slow from what I can see! At 8000+ in colorado SG indicated 10+ This was on the high plain from alamosa to over to the base of Wolf Creek.
As the engine broke in the performance and milage got better. Without the tuner when we 1st bought the rig, trip from prescott az to Lakeside to Tucson best we got was 8 mpg period. Yes some good pulls were encountered. salt river being 1 another is from I17 to payson to showlow.
The wife does not like to dawdle thus the higher speeds. Same deal with my F350 diesel with and without the TT. At 80 with the diesel I get as good as I get trying to go 65-70 and with the trailer at 65 its about the same as 60. Not enough difference to justify the slower speed.
So it will all depend on how you want to drive and how you actually drive the rig it would seem. Got a friend who has a DP drives at 55 claims he get 13 mpg! says he drops to 11 at 65? So again everything is variable. If you ead up on how to economize using CC is recommended as it will maintain the settings better than the right foot will, variations on hills but overall the CC is recommended. I also do not use tow/haul on the rv or pickup except for starts stops on hills and use direct for long pulls and descents.
These are my observations and you need to take them as that and nothing else. I do think tuners help overall but the cost is not something you can plan to recover due to any mileage savings and I do not buy them with that as a primary goal. Both my PU and the RV have rv related tuners installed and we are happy with the results they give.