We, too, recently changed out our 4,000 lb. axles for new 6,000 lb. Dexter axles with 12" brake drums and went up in wheel/tire size to 16" Sendals and Michelin XPS load range E tires. Our two new axles were approx. $1300.
My DH had the help of two friends and they made the change out in 6-8 hours one Saturday. Our friends are seniors, so they don't move as quickly as they used to, nor do we.
Our travel trailer is a Windjammer 3025W. We haven't gone on any long trips yet, but will report later if we have any issues arise.