From the CoachNet FAQ:
Am I protected while in my campsite?
Yes! You are covered not only on the roadside but also while in your campsite as long as you are in a commercial campground or up to 100 feet off of a maintained road.
Obviously other ERS may be different but CoachNet, which is one of the most popular ERS providers, will not cover you if you are boondocking in a non-commercial campsite more than 100 feet from a maintained road.
Good Sam restrictions are a little tighter:
Vehicles NOT COVERED by this program
include but are not limited to:
• Vehicles in a repair facility, off-road area*, sand beach
area, or any unattended, unlicensed, or abandoned
vehicles.
* An off-road area is defined as an area OFF of any of the following: hardpacked or graded dirt public road, paved street, driveway, parking lot, highway,freeway, expressway or adjacent shoulder.
You will notice that it says a GRADED dirt PUBLIC road. This does not mean private roads or trails made to get to a remote boondocking area.
I might as well include AAA road service:
The following services are not provided under the membership benefit:
Service to a vehicle located in an area not regularly traveled by private passenger vehicles (such as a beach, open field, creek bed, or private logging or forest service road and snowfilled or flooded road or driveway).