The measurement specs you need should be taken from your axle.
- Hub face to hub face with the wheel off. (the hub face is where the studs stick out of the hub)
- Inside frame to inside frame where the axle mounts to the frame. (some may want outside frame to outside frame)
-Then there are the leaf spring measurements if you are replacing them.
You might consider bringing it to a trailer dealer and have them measure and order one for you. Then it's on them to get all the measurements right.
If there is a tag on the axle, it might have some axle specs. Perhaps someone know the weight rating of the axle for that camper.