Springs

I'm near maximum weight when all tanks are full and the RV is provisioned and packed with 2 adults.

I did have a leaf added to a previous RV and it made and immense difference.

The tentative plan is to get a 550 from a wrecking yard and swap them with the old tired springs I have now.

The ride is pretty rough already--but I think that is because the OEM springs were pushed to their limits from the get go. I do have air bags but the holders for them require repair.

I won't be doing the work myself--but I have an extended family member who suggested this.

I would go to a spring shop to get the load rating and ride height just where you want them.

Your rig is about 15 yrs old. Not unsual for the springs to sag. I would stay with 450 spring, if you go bigger then the ride will be stiff.
Contact a Spring Shop. There should be one in your town. They will know better than any of us.
They might have fixes that work better.

Air bags are a cheap fix. I use air bags because it is a truck camper that is only on part time, so I can adjust the spring tension. I have been very happy with the air bags.