"and it seems that is the extent of what is allowed for business use of my land"

Let's describe depreciation: an allowance as an expense in consideration for the wear and tear which reduces the value over the useful life of the thing being used.

The campers are on the land, but that's not wearing out the land. The improvements you make in support of the business activity on the land, is what is wearing out over time and would eventually need to be repaired, serviced, replaced. Those are the assets which are in use for the business: Infrastructure to supply water, electric, sewer, fencing, parking or camping pads, these wear out.