What's the maximum slope a porta potty can be placed on without needing special leveling?

A standard portable toilet can handle a slope up to about 3 degrees without needing special leveling. Your 5-degree slope is too steep for stable placement.

On a slope steeper than 3 degrees, the unit will rock, the door might not latch properly, and the contents can pool unevenly in the holding tank. That makes it unpleasant and a tipping hazard. Construction sites use wood blocks or composite leveling pads under the corners to get it perfectly level.

For your backyard, the simplest safe method is to use pressure-treated 2x6 lumber. Cut two pieces about 18 inches long. Place them under the low side of the unit, perpendicular to the slope. Check for level with a small bubble level. The driver might do this if you ask and have the materials ready, but some companies charge extra for on-site leveling. If you don't level it, the rental company may refuse service due to instability or note it as customer-caused damage.

Consider placing it closer to the driveway on flatter ground to avoid the hassle. A unit 30 feet back on a slope is hard to service too, the hose only reaches about 25 feet from the truck.