The F150 HD does have the highest, by far. They are very rare to find new, let alone used.
As for regular half tons, Ford and GM have the highest payloads for crew cab and regular cabs. Ram/Tundra down about 150-200 pounds for the same equipment. Check the driver's door for as equipped payload as you kick the tires.
If this is a second vehicle, why not a 3/4 ton? They are similarly priced on the used market, the base models have all the equipment needed for RV towing, lots of payload, and a good ride when loaded up. Finding a half ton with the right options used can be a chore, while all 3/4 tons have the minimum desired for RV towing. Does it have the towing package? Axle ratio? As a second vehicle the % of unloaded miles is smaller, thus the compromise of half ton ride and fuel economy is not as important as a daily driver and TV.