[Jackflash]

Soils engineering is a science to itself and requires a significant amount of expensive equipment.  The only way I can think of to get a general idea of the bearing strength of a site would be to take a steel rod an inch or so in diameter and long enough to make it weigh 25 lbs or so, weld a small round plate on the end that had an area of about 5 or 6 square inches and drop it from a set height (12 to 18 inches, always the same though) and measure the depth of the indention.  Log the results at every site you visit along with the reaction of the soil to the coach wheel load and after a while you will have a data base that will allow you to pre-test a site before driving out onto it avoiding getting stuck or sinking your jack stands.  Might be handy for all the boondockers.

PS: The bearing area of a coach with a tag axle is only about 2.5 square feet on each side at the rear so if your coach isn't sinking then a jack stand sitting on an 18 to 20 inch square plate shouldn't sink.