Can a restroom trailer be leveled on a slight slope?

A 6-inch drop over 20 feet is usually something the delivery crew can level out, so ask them before you give up on that hill. When we did my daughter's quince on my brother-in-law's back lot, the trailer spot had a similar grade, and the driver set it with the tongue jack, stacked leveling blocks under the low side, and ran the stabilizer jacks down until his level read flat. It took him maybe twenty minutes.

Why they care about level ground: the sinks and toilets drain toward the low end, so water sits where it shouldn't and the holding tank fills unevenly. An unlevel floor also makes doors swing on their own, and the whole restroom trailer can rock a little when people step in, which guests in heels notice fast.

What I would do is send the company a photo of the exact spot with a board and a level laid on the grass and ask if their crew can block it. Also ask about soft ground, because wet grass on a slope can let blocks sink overnight. If they say no, a flatter spot a bit farther away with a short lit path beats a pretty spot that wobbles all night.