A couple of thoughts (and I've towed with gas and diesel and openly prefer diesel):
The transmissions are similar in both, so you could (and probably will) have problems either way. Gas transmissions tend to last a bit longer than their diesel counterparts mainly because the gas engines do not put out the same level of torque at low rpm, but that being said, the gas transmission will have spent a lot more time in a lower gear with the engine at red-line if it did much towing, so it is probably a toss-up as far as longevity.
tvman44 is correct -- costs will be higher with a diesel. Fuel is more, oil, though similar in price, has higher volumes used, filters (if you use GOOD stuff from Fleet) are more expensive, the trucks weigh more so can tend to tear up front suspension parts, etc. But, that being said, I've found that actual operating costs while on the road are less in the long run because virtually every part on a diesel truck is heavier duty than a similar gas model, and I can typically (with my 96 Dodge/Cummins 5.9 12V) double the miles per gallon of the gas trucks I pull alongside of during our pulls to off-road runs.
Total miles driven on diesel engines can also double or more the range of similar gas rigs. On cumminsforum.com guys are reporting total miles on some of their trucks in excess of 1 million! It is almost commonplace to see 500K on trucks that tow for a living.
I've seen 5vers pulled by decent set-up gas trucks, but you are almost certainly talking about a big block or V-10 to move that toy hauler. My brother runs a motocross toy hauler and it is all his Chevy Duramax wants to pull once loaded with tools, bikes, and pit supplies. My Dodge pulls better, but I have the advantage of engine mods for increased power that he doesn't have on the Dmax. My son pulls w a 95 Ford 7.3 Powerstroke, and it also struggles to keep up with my Dodge, but I sure like both the Chevy and Ford platforms much better than the Dodge. If not for the Cummins, I'd have looked elsewhere, but it is hard to argue with cheap horsepower and as of right now, I have only about $500 into my truck after selling the snow plow and I'm pushing 300 hp and 700 torque. Price wise, my brother bought a brand new truck, so he is into that thing for over $500 a month in payments. My son paid $2500 for his Powerstroke, and it is paid for, but he needs a turbo down-pipe, new injector wiring harness, and new injectors -- somewhere in the $1000 range to complete. I paid $400 for mine, sold the plow for $1000, and used the proceeds to start the modification process. My next stage of mods will be transmission valve body, billet torque converter, then injectors and turbo, which should boost me to about 450 hp and over 900 ft. lbs. of torque. I'll end up having about another $1400 in the truck before I make it pretty. Just an example of how it can work for you.
Find a bargain like that and you are home free to fix the couple things the truck needs to give you many more years of service.
