Personally, I'd go for a bigger truck as well. Gas tank size issue set aside, the large truck will be safer -- beefier frame, bigger brakes, more set up for towing than the frontier. People often talk of fuel mileage (MPG) but when towing, the playing field pretty much levels out. I doubt you need a diesel for that TT.
You worry about Ford's ecoboost engines (turbo), and if that is a concern, Ford's 5.0L V8 is a great long life engine. GM and Ram both have great V8's that are non-turbo.
Lastly, going along with my first comment about a bigger truck will be safer, just because you are "under" the tow rating of your Frontier, does not mean it is "safe". I'd bet you'll shorten the life and/or break something quicker with the Frontier vs. a larger truck.
