Look for a tire / loading decal (probably located near the front on drivers side) on the trailer itself. It should show tire sizes and a GVWR for the trailer. It may also give you dry weights for hitch and trailer.
Average trailer load (propane, battery, dishes, pots and pans, camping gear, BBQ supplies, bedding, groceries, water, etc) is 800 to 1000 lbs.
Average tongue / hitch weight is about 12.5 percent of actual trailer weight.
If you find dry weights, add 1000 lbs to trailer weight and calculate 15 percent of the total, for tongue weight. This will only get you in the ball park of your actual weights.
If you only have GVWR, calculate 15 percent of that for tongue weight. This will get you in the ball park of max weight on that trailer.
The ball size is most likely 2 & 5/16".
Your WD hitch should be sized to handle up to the gross trailer and tongue weights.
Example:
If GVWR is 7500 lbs, tongue weight could be as high as 1125 lbs. You would need a hitch rated up to 1200 lbs.
Things to keep in mind:
Tongue weight is not a constant number. It goes up and down during every trip. Different things get loaded for each trip, groceries get used up, holding tank levels go up and down.
Travel trailers can have tongue weights between 10 and 15 percent of loaded weight. Some can be higher. Less than 10 percent is not good. Too lite on the tongue, can cause sway issues.