"Can he order the troops into manual labor?" Yes.
For example, according to the Military Vehicle Preservation Association:
Franklin D. Roosevelt
ordered the construction
of the “Alaska Canada Military Highway" on February 11, 1942.
Approximately 11,000 service men worked exhausting 12 to 16-hour days in harsh
and dangerous conditions to complete the road in record time.
Additionally, there is 10 U.S.C. § 2808(a):
In the event of a declaration of war or the declaration by the President of a national emergency in accordance with the National Emergencies Act (50 U.S.C. 1601 et seq.) that requires use of the armed forces, the Secretary of Defense, without regard to any other provision of law, may undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces. Such projects may be undertaken only within the total amount of funds that have been appropriated for military construction, including funds appropriated for family housing, that have not been obligated
and 33 U.S. Code § 2293(a):
In the event of a declaration of war or a declaration by the President of a national emergency in accordance with the National Emergencies Act (50 U.S.C. 1601 et seq.) that requires or may require use of the Armed Forces, the Secretary, without regard to any other provision of law, may (1) terminate or defer the construction, operation, maintenance, or repair of any Department of the Army civil works project that he deems not essential to the national defense, and (2) apply the resources of the Department of the Army’s civil works program, including funds, personnel, and equipment, to construct or assist in the construction, operation, maintenance, and repair of authorized civil works, military construction, and civil defense projects that are essential to the national defense.