First off, sorry for your loss - as they say, time heals all wounds.

On to your question - it's going to depend a lot on the terrain you're driving in, 8,000 isn't a super heavy trailer but it could feel a lot heavier towing over mountain passes w/ a 5.4. If you're on flat ground the 5.4 should do it no problem. As said before, you won't win any races but you should be able to cruise plenty fast enough. A v10 would do it easier but don't sell the 5.4 short. If you can afford it, I might suggest looking at a 2005 and up truck, the 3v 5.4 found in those trucks has about the same power the early v10's did. And you get the torqueshift transmission as well which is a big improvement over the earlier automatics. First step though might be to determine the actual weight of the 5th wheel, either contact Jayco or post the model here or in the towing forum and see if someone has a similar trailer and can help you.