Sounds like it was matched up well. I have a 09 6.0 and pull about 10k of fifth wheel. Like all small block gas engines, u have to use the correct gear. So I normally lock out 6th gear in Florida and tow with 5th OD. But if I'm crushing around 65-70 mph and rolling hills then I just leave it in 4th and keep the rpm at 3 k. No issues with that, run all day long if I have to, not noisy or straining at all. Actually running a little higher rpm is taking a lot of stress of the bottom end of the engine. Kinda like downshifting when u ride your bike. Easier to climb the hill at a lower gear then a higher gear and lugging the eingine or u if your riding a bike. The 5.3 is a great engine, and yes don't believe the ford 5.4 hype. My close friend had one in his expy, hated towing his bike trailer through the hills. He said it was weaker then his dodge 5.9. Not saying I hate the 5.4, cause I have no experience of with them personally, others on here have and like them. Anyway now he owns a 8.1 in an classic HD. So u can also run the truck using the toggle switch on the shift lever if u have the 6 speed. U can control the gearing much better then gm's computer in my opinion. Any truck regardless will need to downshift or run the hills. Yes I am very aware of the fact that new diesels are powerful and when adding a tuner, well it really moves. But there is an expense to all of that.... Just last week we went camping with my parents and other family members. They own a 2011 3500 Cummins dodge and pull about 15k, stepdad told me that he even runs the hills to keep speed. Yes the truck is stock and it does pull well. One issue though, he got some bad fuel From up there and lost quiet a bit of power. Not a truck fault problem, but lost boost... Kinda slowed him down, and then a regen cycle hit. So a slow go for a while. I guess he's got it fixed now though. If you decide to go larger then a 3/4 ton would overall be better, but it sounds like what u have is already good, u less ur payload has run out. I used to own a 2000 with the 5.3 and 3.42 gears and pulled a small 6500lb if not more fifth wheel. The truck had a 4 speed and pulled in 3rd good, it would just to 2nd gear on every hill. The newer 6 speed has corrected that. When I pull my fifth wheel, I have no problems keeping speed, using the toggle switching. I like the use of it and I keep the thing in the right gear, keeping speed up. Since adding an efi live Blackbear tune, this truck has really woke up. I just run the 89 tune while towing not the 91 tune. The computer has changed now and it will shift at much different points then stock. It holds power much better now over the duration of the pull. So that would be another option for u to look at if you dot mind coming off 600.00. Sorry for long post, just wanted to share my experience.
