That corrosion doesn't look good at all.
Since you are on a budget, the most simple, quick fix for the immediate issue would be to get some thick white adhesive foil which is suitable for outside use, trim to size, and cover the complete area. Scrape off the blisters before and clean the aluminum siding thoroughly before you apply the foil. The skin is not structural.
This will buy you some time and not look too bad.
...but...
IMO, you really need to know what is the reason. If your camper is built like our old Lance, the walls are (from inside to outside):
- a thin sheet of luan (~plywood), covered with decorative plastic foil on the inside
- a wood frame, untreated, stapled together. Voids filled with glass wool
- a thin layer of styrofoam board, maybe 1/8in
- the outside aluminum sheet, very thin (less than 1/32)
For contact corrosion to occur, you need to have an electrical conductor behind the aluminum skin which touches the skin panel. Corrosion will first occur where the skin and the conductor meet.
Normally it is caused by the head of a screw or nail, or a staple which sticks up through the foam sheet. This will be small pinholes which are difficult to detect and can be harmless, depending on their location.
It can also be water saturated styrofoam. Unfortunately, with the large area corrosion shown in your pictures, that is what I would assume. The corroded areas are the in the recesses where the aluminum sheet contacts the foam board behind. Normally, the foam does not conduct electricity and not problems will occur. But when it is wet, it does conduct, and I would expect to find what I see in your pictures.
*IF* you really want to find out, you need to see what is behind the outer panels. If there were indeed moisture to the point that it has corroded through the outer skin, you are very likely to find a much worse structural issue (rot) in the wood frame.
The most likely culprits for water intrusion are the navigation lights at the top, a front window in the cabover (if fitted), the seam between the roof and the front skin along the top front of the cabover, and the outer seams along the cabover edges. Any water that intrudes will collect at the lowest point and soak the structure from there upwards unless it can drain - which it typically can not.
Usually you do not see or small anything suspicious inside for a very long time, as the innermost layer is a watertight plastic foil. And some moisture is always present and can be attributed to condensation.
How to check:
From the outside - To pull the side panel, you need to unscew the corner molding far enough back and upwards to be able to peel it away from the side a few inches. You need to remove the caulking first. Underneath is another type of caulking which is permanently soft. This is the actual water seal - the white outside caulking is more a cosmetic thing and it is only the first line of defense.
The aluminum siding is stapled to the wood frame at the lower edge; you need to pull these staples and then you can peel it back a little.
If everything behind looks good, you can reassemble using the correct types of caulking. You could also decide tom pull a larger part of the wall siding and replace with new material.
From the inside - Alternatively, you can borrow a recirculating saw and cut a small (maybe 3x3) opening in the inside luan, near the affected area. You can then check the inside of the wall for moisture. Cover the opening with a somewhat bigger piece of plywood or whatever you find suitable and 4 screws afterwards.
By measuring device - There are also professional tools which can check for moisture from the inside without any damage or disassembly. A good RV shop might have one. There are also cheap tools (around 10USD) with 2 measuring tips that are used to check if your firewood is dry enough. I am not sure how well they would work in a camper.
