A friend of mine bought a new Montana 30. 5 5er w/2 slides a couple years ago. It was also built by Keystone. He had only had it a few months when he began blowing OEM Goodyears in size LT225/75-16. He replaced all four with LT235/85-16s but the wheels are still six lug and probably underrated for the loads and tires he now uses. A few months later a spring broke, also on the left side. He replaced the broken spring and went on. On his trip home he stopped by to visit me for a few days. On my backyard pad I crawled under the trailer and inspected the axles and found he had 5200# Dexters, inadequate for the weight. When he got home he replaced all four springs with springs for 6000# axles. I couldn't talk him into replacing the axles also. Obviously the manufacturer equipped his trailer with underrated wheels, tires, axles, and springs.
A few months ago I bought a used 5er (see sig). It is a tandem axle trailer. I immediately noticed that the gap between tires on the left side was narrower than on the right side. I had Southwest Wheel (local) replace the brake backing plates, all four springs, shackles, hardware, etc. (surprisingly cheap insurance) When Les lowered it back down on it's wheels I measured the gap again. Still narrower on the left side. We also noticed that the trailer listed slightly to the left on his concrete shop floor. I took it to a public scale and weighed it. I found the left axles are carrying a little over 6000# while the right side axles are carrying just over 5000#. (Two slides, both on the street side plus refrigerator, water heater, and a pantry) I suspect many 5ers have a similar weight imbalance.
Due to the design of the crude old-fashioned leaf spring suspension assemblies most RVs use, as additional weight compresses and flattens the springs, tandem axles move closer together. This occurs because the front axle is attached to the trailer by an eye at the leading end (front) and the trailing end of the rear axle is attached to the frame with an eye also. Conversely, the trailing end of the front axle and the leading edge of the rear axle are attached to a rocker device (don't know proper name). When the springs flatten under weight they can only move one direction, toward the rocker. This allows the axles to move toward each other as much as an inch when the springs are fully loaded.
I took the trailer to a spring shop and had an additional leaf fabricated and installed on the left or heaviest side. This raised the heavy side back to level and widened the gap between tires on the left side. Not a real scientific fix but probably good enough. This trailer has 7000# axles and weighs 14,500 including pin weight. Southwest Wheel will replace the entire suspension with torsion axles for less than $1500. I may eventually do that.
Harvey