The problem with this argument is that the American Revolution, if it was to save slavery would never have allowed democracy but would have created oligarchy (which it did to some extent in the stealth action of the slaveholding elite present at the creation of the Constitution). But democracy appeared and the American Revolution emerged as a republic moving into democracy in a compromise with slavery that a Constitution with a hidden dynamite charge would lead to a Civil War in a generation. The argument that a compromise was forced onto the process seems closer to the facts. Why would a slaveholder republic declare all men were created equal?
We should consider that Britain was not beyond slavery in 1776.1785 (1807, 1833, the relevant dates) and that they also were concerned rightly the new Americans would be genocidal to the Indians.
Again, to see the half-truth only in this argument: consider Athenian democracy: was it founded to preserve slavery? Hardly, there was no challenge to slavery but democracy emerged somewhat chaotically but on its own terms. The Americans looked to the past and the example of Greece/Rome seemed to condition their views.
The University of South Carolina historian holds that the American Revolution was launched to defend the institution of slavery against the British Empire.