The max psi on my pop up camper's tires is 50, What should I put the air pressure at?

50 N0 more than that.

Somewhere on the wehicle, the owner's manual will know there is a tag. Places to look are glove box, door jamb and under the trunk lid for cars.

This is where the inflation pressure is listed for the lunloaded truck. This will be less than the maximum PSI listed on the tire.

Always check when the tires are cold.

The max psi on my pop up camper's tires is 50, What should I put the air pressure at?
I'd set the tire pressure to about 40 psi on all tires and monitor the tire pressure after extended towing.
Inspect your tires periodically for dry-rot and other damage.