If that manual is true and the 5000 lb rating is correct I would think that you are undersized.
I have pulled boats with underpowered vehicles and pretty much ruined all of them.

I can't give you a good technical reason, but I don't think I would ever pull a 4800 lb boat with a 6 banger, regardless of how it was rated (maybe a diesel??)

I would think the mfg assumes people are going to push the rating so they under rate their vehicles, but to be on the safe side I would think that boat/trailer/fuel/gear should add up to no more than 80% of the published rating in the manual, 75% prefered.

My 2cts.
Dave