I know my experience with my motorhome is a little different, but it's all about weight distribution. If you spread the load enough it is less likely to sink. I recently moved my motorhome to my new house and there's nowhere to put it except on the grass/woods. Since we're on a mountaintop with ledge/rocks popping out of the ground I figured it couldn't sink much before it hit something solid. Wrong. It pretty much began to sink immediately to the point I could only see the tops of the leveling ramps. It's a pretty decent slope so I'd never be able to get it out.
I pulled it back out and put pressure treated 2x12s under all 6 tires. That was enough to keep it from sinking. The lumber is barely visible but it seems to have stabilized - situation hasn't changed in a couple of weeks with a few rains.