You need a diesel for that amount of load. It will handle it much better and you have the benefit of more torque for pulling grades and passing very slow moving tractor trailer rigs and better acceleration for merging with traffic, and above all you gain the benefit of the diesel engine's exhaust braking capabilities.
Diesels are more complex and more expensive to maintain but it is the price you pay for towing a 12,000 lb. trailer. The only "complexity" is needing to add DEF or diesel exhaust fluid to a separate tank. Figure on 750 miles per gallon of DEF. DEF is available at the same pumps at truck stops (and some Shell gas stations) as the diesel fuel so it is not a big deal.
All three companies, GM, Ford, and Ram, have greatly improved their engines and trucks and they are far more reliable than the ones from 2010 and earlier in every respect. The only real drawback is the need to use ultra low sulfur diesel fuel which can cause problems if you are in Mexico.