Towing with a 3.73 F250 6.2L, will a 4.30 axle help ??

Seems like Gas mileage is important to you. And you are only pulling 3 times (3000 miles) per year. And your trailer is about 12,500 # loaded.

Based on that, keep what you got and turn up the radio. Let it rev. No...Make it rev. The 4.30's will make a good difference. But for three tows a year? You'll suffer the rest of the year in worst gas mileage.