Portable toilets don't hold much, less than most RVs.
We bought a full sized portable restroom for some property we camp on years ago. I think it was around $400 at the time, but that was like 15 years ago. We call the septic service to get it pumped out in the fall. Holds around 40 gallons, which is a lot when there's no flushing involved.
Call around to the rental places, they may have used ones for relatively cheap.
Here's one, free pickup at your local Wallyworld. Probably better to search locally.