Ok, so the good news is you have all kinds of options, including many that will make it better than new.
Much depends on your budget and time and willingness to tackle a DIY project.
You actually could wallpaper, but it would involve numerous pie cuts to round the corners, and wall-paper may not be the best choice inside an AS travel trailer due to all the moisture and condensation exposure that is so common in a camping application.
Other options include:
- some durable short nap carpeting (like the kind used in entry-ways) that you could glue on that would offer some thermal insulation
- Wood paneling cut into strips -- search the forums here for those who have remodeled the entire end-cap with wood.
- Painting with a good interior paint or Zolatone, a textured acrylic coating AS used extensively in the 60's that offer just the tiniest bit more insulation than paint.
- Sonex acoustic foam panels available in a variety of colors and patterns.
- Bumper stickers you collect from your travels,
- Leather, Pleather, or hides
- Automotive headliner fabric
- Upholstery or other fabric, installed with a foam underlayment for even more padding and insulation.
- Formica
- Reflectix
There are countless other options, no doubt. Have some fun with it and make it your own.