I think the takeaway that the OP should be getting by now is that if you plan on running close to the limit of that 5k rating, it may be better to have the bigger vehicle (v8 not v6).
My personal experience when attempting a similar thing was the 6 was not adequate. We have an envoy xl with the I6...doesn't come with a v6, and for a short pull on flat land it performed very well. The first trip further out did not make for an enjoyable experience. I now have a v8 tahoe that does much much better.
What I found was that the part about going slow with a 6 cylinder...while maybe that only means 5-10 mph slower, made all the difference in the world. Having every car, almost truck/TT combo, and every semi, pass you by in rapid fashion by driving up behind you, changing lanes to go around you, creates like a vortex of air. Even if it isn't creating a vortex, you et zero relief from the constant movement that is created by the pushing and pulling on the trailer that is created as vehicles move past.
And thats not even talking about climbing any sort of grade. I think if your going to stay well under the limits, and you have no plans on upgrading to something larger or heavier, then a v6 could do a decent job. Otherwise, get the v8 from the start, because your going to anyways.
If its simply because you want better gas mileage when not towing, some of the newer v8's get way better gas mileage than the older ones.