sewer smell

".......it only smells when the tank is 3/4 full and leaks under the camper....."

That statement indicates tank is leaking.

You will need to remove section of underbelly OR cut a three sided window in underbelly material and inspect tank.

Until you have a visual it's all guessing.

It will be hard to find if no water is available. You can cut the plastic underneath with a razor knife and tape it back up with tape the color of the plastic. It repairs quite well.

Moved from ATC

Does the unit have a permanently installed tank wash system, that might be leaking, the other is some sensors are also passing through the tank wall and the sensor has a leak on the weld, there may also be a crack, I do not know how the tank is mounted to the RV and something moved and punctured the wall.

You are going to have to remove part of the under belly and actually inspect the tank, my suggestion is to clean the tank as much as possible and use clean water to find the leak, if you dilute a little bit of engine coolant and use a ultraviolet lamp at night you will detect the leak as you fill the tank.

The ultra violet lamps are called black light and are sold at hardware stores or nurseries.

Good luck with RV park perfume.

navegator

Check the roof vent.