This looks like many of the older campers. I am in the process now of repainting my frame, just it is not as advanced as yours. Mine was made in Nov, 2003. The original paint job while good for the RV industry, was not very thick paint. After 10 years it really starts showing the rust and if it is has been towed in the rain and lives outside, it just gets worse. Any type of road salt from spring camping trips, even from wet roads plays havoc on the frame. The only way to stop it, is keep after it with better primer and paint.
To your questions, we cannot tell every joint from here based on the pics. I would suggest this as a place to start.
At the suspension area, check every bolt, shackle and the U bolts. If they look compromised, just replace. If the trailer was towed a lot, they may be worn bad as much as they can be rusted bad. The parts are not that expensive. Odds are high the thin plastic bushings in the spring eyes are worn out gone. Even the new campers have them worn out in less then 10,000 miles.
The spring hangers that are welded to the frame, check these for cracks or extreme rust that have large metal loss. They take a good deal of flexing when the camper tows. If you find one really bad, they can be ground off and new ones installed. If you get to doing that, lets talk on it so the axle alignment is done correctly when installing it.
This goes for all welds, especially on the frame header and the A frame area. Corrosion can start at the heat affected areas first and may be more advanced then the open frame. Again you are after extreme rust that has a good quantity of missing metal.
What you have right now looks bad as there is no paint left. Some of it may be bad in certain areas, other areas it just needs to be reconditioned.
Guys and Gals in old car restoration deal with this all the time as many old cars and trucks look even worse then the camper frame. Searching in that area can help on what they do.
What I have done is this. My rust is not as advanced as yours but the same process applies.
You will need to putty knife scrap the heavy rust barnacles off. They will come off fairly easy. From there wire brushed help get the finer rust. Do you have a 4 1/2" angle grinder? They sell wire brush wheels that can go on them and they do a good job of cleaning it up. Your not going to get back to bare metal, but try and get any loose surface rust off so what is left is hard and stays put.
They help on the big areas, the hand wire brush still needs to fit where they cannot. Once done brushing, you need to clean the metal with a degreaser and cleaner so what ever you out on, can bond to the metal/rust.
What to use, as protection covering. I'm learning my way through this now myself. A few years from now I can report how this is doing, but I do not have good long term data.
They make rust converter products, it chemically converts the rust on the metal into a non rust surface. It stops the chemical rusting action. I have used the Eastwood converter with mixed results. Where the metal is heavily rusted it seems to work better then mildly rusted. It is like it needs the porosity of the rust to stick to it. It does not work well on shiny clean metal. There are other brands of converter, the key is how deep does it penetrate. The concern is rust bleed through if it has not totally sealed the rust. I know Rust-O-leum makes a converter but I have not tried it.
Rust-O-leum makes what they call, "Rusty Metal Primer" in their professional grade paints. It is an oil based product. This goes right on the rust directly. I have used a good qty of this, and I like the way it works going on, it sticks, it covers well and bonds very good. I do not have long term data on if there is rust bleed through yet. They make White metal primer too that goes on clean metal or slightly rusted. This may be a consideration if you are going the converter route first.
To top coat, I have used Rust-O-leum professional black gloss paint. This is oil base paint and is a very good paint. It create a tight bond and cures hard. However it is the top coat. What is under it is the biggest concern of rust bleed through.
POR 15 is a product that many have used, I have not so I do not know details on it.
Others on the forum have done some of this too, they may chime in with their experiences.
You will need to pull the brake drums and go all through the brakes and bearings repacking with fresh grease and new seals. Make sure all the parts are pivoting and the adjusters are not froze up. The brake wiring may be old and brittle and need to be replaced. You will have to check and make a judgement call on it if is good or not.
The wire that crosses inside the axle tube can be the worst. It is a text book classic that the wire in the tube becomes very brittle on the insulation. Bouncing down the road, the insulation wears off or breaks off and then shorts out the brake circuit when you apply the brakes. The way out of this, just install new wire (2 wire cable) on the back side of the outside of the axle tubes. The back side is preferred so weeds and things do not ding up the wire driving forwards. Or run new wire down each side of the camper and jump to the brake coils from each side.
Hope this helps