Congrats on your new Sunny!
Sounds like someone was having tire issues and threw whatever they could find on there to get it going. At this point I would not trust anything they did and you should look at and measure every rim along with the bump clearance on the camper. The below info can help sort this out.
Your tires, you might need "all" new tires and possible some of the rims depending on how goofed up this is.
First off are the age of the tire. There are DOT date codes on the sidewalls which tell the month and year the tires where made. Trailer tires should be replaced every 5 years due to the tire rubber itself degrading and not due to actual tread wear. This link shows how to read the 4 digit code https://www.tirerack.com/tires/tiret...jsp?techid=11&
Running old tires ST trailer tires is a tire failuire waiting to happen.
Note: They only put the date codes on one side of the tire, may have to look on both sides to find it. In the link above the date code is 5107 meaning the 51st week of 2007.
Now to the size. All four tires including the spare need to be the exact same size and load range and all be radial tires. No mixing of a bias and radial tire. The ST205/75R15 is a very different tire then a ST225/75R15. The outside diameter is different, the width is different the the load carrying ability is different, even the rim width that both are supposed to be mounted on is different. And...then there is a load range on both sizes that can change. Someone stuck a different tire on there to get it going but it is for sure not correct.
The next question what is the correct size? According to the 1988 brochure the camper came with H78-15-B tires on it and the GVWR of the camper was 6,440#. That size is the old standard and they are no longer sized that way.
This first starts with what size rim do you have? While it seems like all of the tires may be 15" there are rim width differences. You can measure the rim width with a tape measure. The part you are measuring is the steel part that the tire bead is mounted too.
ST225/75R15's are supposed to be on a 6" wide rim.
ST205/75R15's are supposed to be on a 5.5" wide rim.
See which you have and report back.
The next is the load range. On the side wall it should state the load range and the "max air pressure cold" air pressure.
Both of those sizes come in
C load range, which is a 50psi max
D load range, which is a 65psi max.
Pending the size, the weight carrying ability will change with the load range.
The next thing is tire wheel well clearance. With those ST225/75R15's on there if the wheel wells where made for the ST205/75R15's, then there can be a bump clearance issue. Meaning the tire can bottom out in the fender well going over a large bump. That = damage coming very soon thereafter.
Check what you have now on the ST 225's and the ST 205's. You "have" to have at least 2.5" of bump clearance our you will hit. More is better but Sunline sometimes set these up as lower riding campers and set them up less the the 3" Dexter axle recommends. Odds are high the ST225's may hit as they may not have the correct bump clearance.
Someone may have thrown a mix mash if tires and rims on the camper. At this point you have to sort out what rim size, bump clearance size you have on all 6 rims. And all 6 may not be the same either....
For a weight rating of a camper with 6,440# max loading, this could line up with using ST205/75R15 Load range C's. These are mounted on a 5.5" wide rim. This would be a 50 psi tire, each tire can support 1,820# (all 4 are 7,280#)
While they do offer a ST225/75R15 load range C tire, it seems too big for that camper. The pressure would be 50 psi, this needs to be on a 6" wide rim and each tire can hold 2,150# or 8,600# total weight.
I'm thinking someone may have either overloaded the camper above the 6,440#, had old tires, ran them low on air etc or all of them at once and had blowouts. And threw ST 225's at it to trying fix the issue rather then trying to understand why they were blowing tires. And they may have not looked at the rim width even or the bump clearance.
Hope this helps. You have some sorting out to do.