Not sure where the Motorhome fits into the discussion of new / diesel pickup. In any case....I've used 2-3 diesels for a couple of TT's and a 5th wheel as well as the current F250 diesel for my non-RV towing. I'm not interested in being the 1st guy up the hill, but I do like to make it up the hill w/o seeing lots of flashing lights and warnings. So I'd recommend a diesel if you're getting a new tow rig.
That being said, routine maintenance is more expensive on a diesel and the infrequent "catastrophic" failure (e.g. injectors) is very expensive. All this would roll into what a couple others have suggested: if your current rig gets the job done (and is paid for), probably cheaper to keep it rather than chase a new vehicle.