Someone mentioned taking off the wallpaper border. That's going to be the quickest way to neutralize the space and let you bring in your personality. I guess it also depends on the wallpaper. Ours is pretty basic/cream.
Next curtains/valances. Depending on the construction, you could use hot glue or a staple gun to cover valances with a different fabric. If you're not in love with the blinds that come with the RV, consider switching to a more traditional style blind. The blinds in my home are wooden, birch I think because they are extremely light weight. In all the RVs we walked into, those with these blinds felt the most homey to us.
You could also install pull down shades and simply cover the shades with a favorite fabric.
Don't underestimate the power of paint, fabric and a glue gun.
Use a bright, fun contact paper (wallpaper or even wrapping paper with clear contact paper on top) to line your drawers and cabinets.
Slipcovers can work wonders on chairs and sofas.
If you don't love your recliners or dining chairs, put them up on craigslist and find something better. We ordered our RV without recliners and dining chairs. In our floor plan, the standard dining chairs are like a big wall between the living room and kitchen. So we're looking for something with an open back or bar stools.
Add a pretty backsplash. There's a great thread here on the boards where a few folks have added tile backsplashes to their kitchen. There are also gel-tile stickons you can use. I haven't seen them in person, but if they look as pretty in real life, I plan on adding them to my kitchen.
Depending on what knobs/pulls you have in your kitchen and bathroom, you might consider switching those to something brighter or maybe more reflective. I've never seen a Heartland with the gold knobs, but if that's your thing, you can probably find them for a dollar or so a knob at Home Depot or Lowes. Same for silver or brushed nickel.
Bright new towels can really pep up a space. Think bright whites along with a fun accent color.
One quick idea for changing up your space is to go with a single accent color. A popular color this season is yellow so you could do yellow accent pillows, a yellow throw, yellow dishes, a yellow rug and maybe a yellow painting or sculpture. Just little pops of color on a neutral background will make your space feel very pulled together.
One thing I've been wondering about is changing the linoleum in the kitchen. Right now I like the color/style that's in our unit. But it's very dark. And the cabinets are pretty dark. So I have a feeling at some point in the future, I'm going to get tired of that darker floor. I'll be interested to see how hard it is to change.
If you're feeling more adventurous, here are a bunch of different pictures from people who have stripped their RV of wallpaper and decorated pretty much from scratch:
An example of bringing in a central accent color -
http://pinterest.com/pin/232568768227342712/
http://pinterest.com/pin/232568768227342710/
http://pinterest.com/grammajay/rv-ideas/
Love this pillow -
http://pinterest.com/melaniesantizo/rv/
All sorts of ideas and color schemes -
http://pinterest.com/tdsprague/for-the-rv-park/
Colorful campers -
http://pinterest.com/kbandaruk/rv-life/
Cute RVs -
http://pinterest.com/charleyj/rv-cutes/
Some custom "cafe" style curtains and a great headboard idea -
http://pinterest.com/papes2dr/rv/
Can't believe this bedroom is in a camper:
http://pinterest.com/pin/220465344228786557/
Paint on the walls & cabinets, new backsplash -
http://pinterest.com/pin/130182245449071465/
Hope that helps!