Owned two TTs and two motorhomes. I've traveled at 55, 60, 65, and 70+. Don't much like anything over 65, and don't feel the need to do so any longer. Have always felt stable and remained very attentive at any speed, but don't ever want to get involved in any accident at any speed. I know my reaction time isn't what it used to be (was lightning fast, but not any more). So, 60-65 is plenty. Doesn't much matter what sort of rig it is, as heavyweights all have longer stopping distances and limited maneuverability. Currently, our Seneca and Jeep LJ combination has plenty of stopping power, but I feel no need to push the limits. Your best fuel mileage will be in the peak torque range, whether gas or diesel. That's the sweet spot for flat ground. I have found diesels to be happiest just under 2k rpm for peak efficiency (1700 to 1800 or so). Gassers a bit higher, maybe 2500 or so. Whatever gets you there in one piece with enough left over for a few nice dinners and a cold beer is good.
2006 Jayco Seneca 34SS
2006 Jeep Wrangler Unlimited Rubicon Toad