The upholstery on vintage trailers doesn't last long. But it's simple upholstery. The great thing about redoing the upholstery is that you get to put your own fabric in.
This couch is made from a bottom cushion and 2 back cushions connected with 2 hinges allowing it to be made into a bed.

As you can see, the base is made from a simple frame work. It's structurally sound so I won't be doing anything with it.
First I'll start by cutting a piece of 1/2" plywood for the bottom. I'm putting cushioned wall panels in so I'm cutting the bottom 1-1/2" short.

A good pair of upholstery scissors will make your job easier although the cheap ones you have will work. An electric bread cutter works too.
Next lay your plywood on your foam and draw a line around the board so you can cut the foam to shape. If you're just redoing the existing upholstery, your foam and board are already cut.

Then cut out the foam. Cut right on the line keeping the cut as straight as possible. Imperfections will show through. Make sure the cut is horizontal. It'll look lame if it's sloping and sloppy.

Next lay your foam on your fabric. My foam wasn't big enough for my bottom so I'm adding a section on the back to fill it up. If your foam is 4" thick and your board is 1/2" you need 4-1/2" plus 2" for attaching the fabric to the board for a total of 6-1/2" on each side. Draw a line around your foam and measure out 6-1/2" on each side for your cut marks.

After you've cut out the fabric, you need to cut the corners out but, you need to leave an inch to sew the corners together, so draw lines on your corners as in the photo. You'll then cut out the area with the X.

Another alternative to sewing the corners is folding the fabic around like a christmas package. and stapling it to the back. If you put the folds where they're not visible it works real good.

Now put the pieces together and position everything so that it fits evenly on your board, on all 4 sides. The trick here is positioning it so that it is centered and the fabric is fitting snug and even on all sides. Then turn the whole thing over to staple it.

You can now begin stapling the fabric to your board. You don't want to pull it so tight that it leaves creases where the staples are. But it should be taught.

Just staple about every 6 inches as you go around it.

I used the same procedure for the back rests. They were the same width by 10" each. They were then connected together with 2 hinges.

I used the same procedure for the wall panels and the side seats except I used 1/2" carpet pad.

And the finished product was quite comfortable. I paid about a yard for this fabric at a retail fabric store. I used to shop at wholesale upholstery suppliers and would've paid for the same fabric.