Our property slants from front to back, so I had to have a parking pad beside the garage framed with landscaping timbers and filled with fill dirt/sand/crushed rock (bottom up in that order). One of those large compacting machines - self-propelled, many tons, jumps up and down - was used to compact the fill. My wife thought we had moved the house to California when it was being done.
The boat that preceeded the travel trailer weighed about the same - 6,700# gross. Both trailers are/were dual-axle with decent size tires. Over a couple of years' time, I found that the tires further compressed the fill and/or pushed the crushed rock aside. In addition, it was very painful getting down on the crushed rock to work on anything low or underneath either rig. As a previous poster stated, I was also concerned about what the sharp edges on the crushed rock were doing to the tires. I finally bit the bullet last fall and had concrete poured. It was expensive, but I'm very glad I did.