Pin weight and hitch weight amount to the same thing. Pin weight normally refers to a 5th wheel trailer, hitch weight would normally be a travel trailer. In both cases, it is the amount of trailer weight carried by your tow vehicle. 5th wheel pin weight would normally be 15% to 25% of total trailer weight. Travel trailer hitch weight would normally be 10%-15% of total trailer weight.
Payload is the difference between the trailer dry weight and the max loaded weight (GVWR). For instance, my trailer has a advertised dry weight of 10,450. The GVWR is 13,850, giving me a payload of 3400 pounds.
It is a great idea to actually weigh your trailer and truck when it is loaded for travel. Most Rvers use the CAT scales found at the truck stop. I pulled my rig on the scale so that the front wheels are on one weight pad, the back wheels on a 2nd pad, and the trailer is on the third pad. After going in and getting this weight (they charged me $10), I unhook my trailer, go back with my truck, and ask for a re-weigh ($2). Here are the numbers from a trip through the scales earlier this week.
steer axle - 4480
drive axle - 5960
trailer axle - 10820
gross weight - 21260
steer axle - 4500
drive axle - 3660
gross weight - 8160
From this my hitch weight is 5960 - 3660 = 2300
My trailer weight is 2300 + 10820 = 13120
Given that my trailer is rated for 13850, I have room for another 730 pounds of payload.
My pin weight of 2300 pounds is 17.5% of my total trailer weight.