What are you trying to tow? I think your 500 lb trailer capacity is way off. There should be weight capacity info in one of the cabinets, have the owner send you a picture of that info and the weight rating info off the platform hitch might save you a long ride.
I would be concerned with max hitch/ball weight as the overhang on these vehicles is great and hence the rear most weight really effects the driving control in crosswinds. You can minimize the water you are carrying if the tank is at the back (rearward of the rear axle) and gain some ball weight capacity. If the trailer is a tandem axle, you might control hitch weight with trailer loading balance. I suggest trailer brakes, and that will require a 7 terminal connector unless you run your own circuit. Either would require a trailer brake controller, unless you trailer is withing the max GCWR. I appreciate this is general info and does not specifically answer your how much questions, but best I can do. Also gasoline weights 6.8 pounds/gal and the if the tank is between the axles, can gain some load capacity by restricting the total fuel your carrying. Its all a big trade off if your near the max rating for any parameter. You will need to monitor rear axle GWR more than GVWR on these vehicles.
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