Can a porta potty be placed on a sloped driveway?

Yes, a standard unit can sit on a driveway with about a two-inch drop over eight feet if the crew levels the skids on delivery.

That grade is mild. What "level ground" usually means in practice is the cabinet sits flat enough that the door closes cleanly and the holding tank does not list hard to one end. Drivers put wood blocks or shims under the low-side skid until a level shows the floor is true. Once it is sitting flat, tip risk on that kind of slope is basically zero for a normal residential rental.

Liquid will still sit in the tank as designed. Slight out-of-level before shimming can make the door swing open or shut on its own and can make the seat feel weird, which is why they fix it at drop-off rather than leaving it tilted. For a three-month stay I would still ask them to confirm staking if your street gets wind or kids hanging around the unit, but staking is about tipping from outside force, not about that small driveway grade.

Since truck access is already fine, you are past the usual deal-breaker. Tell them the slope measurement when you book a standard unit and have them level it before they leave. Weekly service works the same once the unit is sitting flat.