The Packers trailed 22-0 after the first quarter, before striking first in the second.
Christian Kennedy found Payten Mansfield for a 5-yard touchdown to narrow the gap.
The Vikings answered just 10 seconds later to go up 28-6 heading into the third.
The Packers were susceptible to big plays throughout the night. Tri-County scored on plays of 75, 34 and 44 yards in the second half.
Leading Fremont offensively were Tristen Marfia (10 carries for 42 yards), Christian Kennedy (2 catches for 47 yards, 1 passing touchdown), and Cody Andree (3-9 passing for 49 yards).
Kyler Kolk paced the Packers defense with five solo tackles and three tackles for loss. Mansfield and Marfia both had three tackles and one tackle for loss each.
Fremont falls to 0-7 overall and 0-6 in conference.