Like already mentioned, not much difference between driving a 36' vs. a 40'. Either way you just plan your fuel stops, rest area stops, groceries, etc. but it will be the same with either and no much trouble at all. We went from a 30' gasser to a 38' diesel pusher then to a 40 footer and there was a bit of a difference between the 30' and the 38' but marginal and no real difference between the 38' DP and the 40' DP. One thing that will be noticed however, is the distance between fuel stops on the 36' gas vs. the 40' diesel as I found I wasn't stopping for fuel nearly as much so the anticipation of where the next fuel stop was diminished to almost nil.
I also prefer the braking of the air brakes compared to the hydraulic but that's just me. As far as camping spots, I have found that most that state a max. length of 35' I can still put my 40 footer into. When we went from the 30' to the 38' we did however, change our destinations a bit and don't stay at state and/or National Park campgrounds as much as we used to but we don't miss that so you will maybe want to check yourself on that front.
Personally, I would take the chassis, brakes, ride and handling of the diesel over the gas but it will also depend on the type of RV'ing that you do. If you travel a lot you may like the DP more, if it's on a decent chassis to begin with, but if you go to one place and stay a while and don't put many miles on you may find the gasser better suits your style as there will be a little less costly maintenance and upkeep compared to the diesel chassis.
Mike