Broken axles

You definitely need axles rated at more than 3500 lbs each. The 3500 pound rating is with weight evenly distributed. Unfortunately on a travel trailer one side can be heavier than the other and the front and the front heavyier too. Therefore, one half of an axle can be carrying more than its share of the weight, especially since you had 3500 pound axles and your weight rating of your trailer is over 7500 lb. You should definitely upgrade with replacement axles to a higher weight rating. The manufacturer when putting the axles on was relying on the tongue weight of the trailer using up part of the gross weight of the trailer bringing the actual load on the axles at just under 7000 pounds total. this doesn't help much when you consider one side of the trailer putting more load on half the axle then it was designed to carry. Not to mention shock loads to the axles created by impacts with potholes and other road debris. Manufacturers cut it too close when putting on axles to save a couple hundred dollars each for a heavier rated axle. We're not talking hundreds or thousands of dollars difference in price for better axles.