Nobody in their right mind will say that a gasser will tow an 8500# Trailer as easily as a CTD. But I think it would be equally ignorant to claim that a gasser WON'T tow an 8500# trailer! That's really my point. Now, once you unhook your trailer and need to drive it the other 99% of the time, what will be better on your back?
On page 10 of Trailer Life magazine, Nov 2010 issue, I just read that Ford will offer a 3. 5 V-6 "ecoboost" engine in their F 150 Crew Cab model that is rated to tow a 11,300# trailer, and gets up to 20% better fuel economy than a V-8 with "comparable appointments"( whatever that means )Another magazine claimed to have used a Ford Flex with the same engine to tow an 8500# trailer and "barely noticed it behind them. " (THAT one I have a hard time believing). I feel an 8500# trailer behind my truck! If I didn't feel it, I wouldn't be very observant, would I?
So, to go back to what the OP asked, yes a gasser will tow it. Yes it will work harder. Yes, it will use more fuel in the process. Yes, it probably won't go 1,000,000 miles.
If your back is that bad, and you are considering a truck that offers a softer ride, then there are options. You can soften the ride of your current truck, or buy another truck that rides better.
Transporters use diesel engines for a reason, they tow a lot. You don't. So you don't necessarily NEED a diesel. Like I said before, test drive all options, and get what fits your needs the best. Just keep an open mind, that's all I'm saying.....