Robbie,
When I was getting ready to put one in our previous 2955, I was limited to the width and had to go with a 23" like the ones here by classic Flame or Dimplex:
http://www.electricfireplacesdirect....PROD&ProdID=64.
The Dimplex model requires you to build your own trim system of metal, tile, wood, or whatever. The Classic Flame has a black trim already.
The 26" Larry shows is ideal, but I never pursued it far enough to see if it would actually fit in the cabinet with significant modifications. The 23" will drop right in. I see now they even have an 18".
Have fun and enjoy it. We ended up just getting a new trailer with a fireplace, so I never did install one in the 2955. Had the fireplace on a week ago tonight when fog rolled in the drizzle and temps dropped to 40. ahhhhh - makn me sleepy