I just purchased an old camper with a 50 AMP male plug to connect the electric. Currently I have used adapters to bridge 50 amp to 30, then 30 to 15 which I have plugged into by electric box with an extension cord. This works for fridge but is not working for the stove or water heater. I need to hard wire the camper to the box, but am not sure how to about doing this. PLease help.

You will need a cord that's rated for 50A and connect it to a dedicated 2-pole 50A breaker in the box, as well as the neutral and ground.

This can be a dangerous thing to try. If you are not comfortable with this sort of thing, please find someone to help you.