It has been my understanding that the condensation problem was due to the intake air cooler being too efficient, and that they'd corrected it. Plenty of happy ecoboost owners out there, as someone pointed out. That said, I think the 5.0L is an excellent engine also. You could be happy with either one.
As for Ram 1500 diesels, no you aren't likely to find a used one. And they have some special requirements, like low-ash synthetic oil and diesel exhaust fluid. They might get you 16-17 mpg, but the higher diesel fuel price and higher initial cost offset that to a great degree.
I wouldn't hesitate to tow that trailer with an F150. And there are a ton of used ones out there, which helps with the pricing.