Just because an oil is synthetic does not make it a better oil for the projected use it will see. My personal opinion is to save that oil for the winter and get some M1 10W-30 HM for this oil change. The anti-wear additives will be much higher for the hot, towing season, when these additives will come into play more. And the higher viscosity of the HM 10W-30 can only help in this situation as well.. does not matter if your engine is a "high mileage" engine yet or not.
A 24ft. trailer is no small potato! Your engine will benefit long term from the beefier oil.
10w30 is NOT THICKER than 0w30 at 100C (212F) where the specification benchmark is done. As the temperature goes higher than that, the 0w30 may actually retain viscosity better because it will have a higher VI (and is typically a better base oil).
You may be right about the add pack, but frankly everything I've been reading lately indicates that the 0w30s and 5w20s are some of the best oils out there. Better base oils, good addpacks. Great UOAs...
You might also consider an HDEO in 5w40, like Rotella T syn, Mobil 1 TDT, etc.