$2700? Ouch. If you have that good of tire wear then IMO the axles are fine. I know from experience that a bad axle will wear a tire out rather quickly. If you're somewhat handy then my suggestion would be to raise the tank back up and install a 2"x2" piece of angle iron across the center of the under side of the tank. Bolt it up to the flange on the frame. My 5'er has that and between the edges holding the tank and the angle iron under it, it's not going anywhere. You may have to have the side support/s rewelded but that would be relatively cheap compared to what that dealer wants.