Oxidation is what wears out transmission fluid and oxidation is caused by heat. If the transmission fluid temperature never exceeded 170 degrees, it could in theory last 100,000 miles but if the temperature routinely exceeds this by 20 degrees, the oxidation rate doubles.
So at 190 degrees the replacement mileage would be 50,000 miles. But when towing a trailer through the mountains transmission temperatures can reach 210 to 260 degrees. If this type of towing was routine, the fluid change interval would be 12,000 miles. Stop and go driving also increases the temperature of the fluid.
If you had to rock your vehicle out of the snow numerous times the fluid may be oxidized.
Oxidized fluid no longer lubricates the transmission sufficiently and will cause excessive wear.