Hi WannaSunline,
I will pass along some mechanical info concerning increasing tire sizes to try and help.
1. Bump clearance. This is the distance between the top of the tire to the bottom of the inner part of the wheel well. This distance becomes critical when you go over a large bump and the suspension does a large amount of flexing and hopping from the bounce. Dexter Axle one of the big names in the industry recommends 3" bump clearance when the camper is fully loaded. Meaning the springs and tires are compressed to the axle loading rating.
In my dealing with the newer Sunlines, Sunline would set up the campers as low riding campers to help make them easier to tow with less wind drag for lessor power tow vehicles. I have found that Sunline has used 2 1/2" as a bump clearance. And I have measured this and less to see if it will work in all conditions. In my tests, the 2 1/2" will work. 2 1/4" can hit in some cases. I would not recommend anything less then the 2 1/2" due to the potential of damage of the bump clearance.
2. Wheel spacing. The distance between the 2 tires on a tandem trailer should not be less then 1" when the trailer is fully loaded to the axle rating. If you go less than this, under certain conditions and flexing of the suspension there could be a hit. Suspension wear over time can aggravate this clearance making it smaller.
I also have found on many of the newer Sunlines that the springs Sunline picked to setup the suspension are shorter than the industry standard which in many cases is 33" as a standard on the larger campers. I really do not know why they did this but it makes running into the 1" spacing need between the tires all that much easier.
Some of the low riding Sunlines have had issues taking on larger tires as they exceed item 1 or 2 mentioned above. Some members have done what is called and "over/under axle conversion" often referred to as a axle flip, to increase the height of the camper off the ground. They normally do this to stop dragging the back end of the camper going up and over bumps, driveways etc. However doing the over/under conversion is one way to add extra bump height if you are changing tires sizes. It does not help the tire to tire wheel spacing issue but it does the bump clearance.
When Sunline changed tires sizes on a certain model they may have changed the wheel wells to go along with this to allow the bump clearances and tire spacing to work. I am not exactly aware on what models they did or did not change the wheel wells but it may explain why some years with larger tires work on a specific model and some years will not. Point being, check and make sure the new tire OD's will work.
In my case I had 15" tires and needed to go to 16" to upgrade the tire to LT tires. I am in a very different weight class then yours but I ran into all the issues above trying to do so. I had to rework the suspension and trailer ride height to make it all work. It can be done, just not always simple or cheap.
Hope this helps and good luck with your camper. Let us know how you make out as others coming behind you can benefit from this info even if is that it cannot be done because of some reason.
Thanks
John
