Q.Am looking to define what "mantross" means as it is listed as the rank of a relative who fought at Valley Forge.
A.I believe what information you are looking for is the definition of a "MATROSS" rather than "Mantross." A Matross is a soldier who assists artillery gunners in loading, firing, sponging and moving the guns. There were many of these men encamped at Valley Forge during the winter of 1777-78 and were "retrained" with the arrival of von Steuben. While certainly not a glamorous role, it was very essential!
SAS, Courtesy The Valley Forge Historical Society