You might consider getting some of those straps that go around the frame of a mobile home and attaching them securely to the fiver's frame. Then attach those to 4 mobile home anchors, the type that will auger in the ground, one at each corner.
For a more secure anchorage, it'll be necessary to support the trailer's frame on blocks so it wouldn't rock on it's springs. If you're just trying to satisfy the campgrounds requirements, I wouldn't worry about the blocks.
for a site telling how to install the tie-downs and the anchors.