Welcome to the world of "trying to figure out what my truck is rated to carry". Hats off for doing your homework, most don't ask these questions until after they buy a trailer that's too big....
Some points may help: The 150-driver issue...there is no allowance for the driver in the payload number. However, most mfrs include a 150-lb driver in the calculations when quoting a tow rating.
On the payload, you mentioned that you looked up your specific model, which is good. But the payload quoted is going to be the weight of that model subtracted from the GVWR (7200 lbs). The issue becomes when you add options that aren't included with whatever the base trim level is for that model. Things that add weight could be towing package with HD radiator and trans cooler, bigger engine, or luxury items like leather and a nav system. So for example if you look up the payload for the "SE" model, it will have taken into account all the stuff that comes with the SE model, but not the options that you can add over and above. After all, the mfr has no way of knowing how each vehicle will ultimately be configured, there are too many permutations. There there are the aftermarket items like the tonneau cover, etc.
So you really not too far off the mark from the brochure payload # everything considered.
shorteelaw wrote:
But giving ourselves 100 extra pounds of payload would give us approximately 660 extra pounds to have in the trailer, right?
Yes. In general a travel trailer will have a 13% tongue weight, so for every 100 lbs of available payload, 100/.13= 769 lbs of total trailer weight. Of course this number will vary depending on how the trailer is configured.
So, some of that depends where on the trailer you add weight. An extra battery on the tongue will add almost all of it's weight to the truck's payload. A case of beer directly over the trailer axles adds no weight, and any weight added behind the trailer axles actually reduces your tongue weight and increases you available payload.
If the fresh water tank of the trailer is behind the trailer axles, you can actually reduce your tongue weight and increase available payload by traveling with a full water tank. However, you need to be careful that the weight of the water doesn't put you overweight on the trailer axles.
Hope some of this helps.