I am going out on a limb here but I think most of us with smaller trailers that don't actually "need" a diesel have a diesel because we want one. We didn't have a camper when I bought my GMC in 06, but I bought the diesel "because I could". And I did have other plans.
In the pickup truck market, if you are thinking of buying a diesel for the fuel savings you are wasting your money.
But for towing it can't be beat, even with a small trailer. Nothing like setting the cruise for 60 on those long hills on RT. 81 and having the truck stay at 60.
The biggest maintenance difference is the amount of oil, typically 5-6 quarts versus 10-14. Fuel filters cost more, maybe the air filter too. Even in New Jersey, with the highest auto insurance rates in the country the diesel is not an issue.
Diesel for no other reason then corporate greed costs more then gasoline, but the mileage offsets the difference.
I have no idea what a Sundance XLT255RK is, and the new Heartland web site didn’t come up with a trailer.
If it is a travel trailer and you really like the RV'ing thing the odds are good you will upgrade to 5th wheel sooner rather then later. In that sense it will be more economical to get the diesel now rather then buying two vehicles down the road.
I bought my GMC HD Duramax in January and I was very happy with it. We bought a used travel trailer (29’ Sunnybrook) the following summer. Liked the RV thing saw we bought the Cyclone in the summer of 09.
The 2500 has become less then desirable le for the task. It pulls fine, stops fine but the trailer is too much for it. So even though we ended up with the diesel in the beginning we are now getting ready to order a new diesel, only this time a 3500.
My opinion, unless you really need to save the money, buy the diesel. I don’t think you will be sorry. And you didn’t mention whether you were talking new or used for the truck.
If used, that is a “whole ‘nutter ball game”
My 2 cents