Our new trailer is aboot 1500-2000# heavier than our last Jayco, and our Excursion does just as well with it as it did before. We get 7.5-8.5mpg which seems to be fairly consistent, even with headwinds. Only travel around 55-60mph most of the time. We have 3.73 axles and would love to go to 4.30 or even 4.56 ratio in the near future. Ours is a 4X4 and would recommend it to anyone. We camp in the woods and have needed it more than once.
As to the diesels, personally I would say that a V10 gas would tow just as well or better than a stock 7.3L. A 6L definitely has more power, but does have a poor repair history, although the 2005's seem to have less problems.
Previous tow vehicle was a Suburban with a 7.4L with 4.10 axles, and I can say that the Excursion does a better job, slightly better mileage, and more power. We regularly run mountain passes up to 7500ft elevation, and while not "flying" up the hills, we are not a 25-35mph road obstacle!!!
Think you would like an EX, and as mentioned, you can buy a "new" one. Providing you have LOTS of cash!!!! Usually in the 100K area when all is said and done!!!