Our cheap Sanyo Vizon TV shown has since bit the dust (it was 5 years old and traveled many miles). I slung together a cheap scrap material mounting system since I was very unhappy with the one I had bought (after only a couple of months, I ended up having to bungie cord the thing to the wall in the Class C while traveling). We will replace old broken TV with one that will fit in the space the old one did. Based on measurements, that would be a 32" diagonal (some not all) as I have a limit on the height. Since we figure TVs only last a about 5 years anyway, we will go to Wal-Mart and buy one with the best picture, that fits in our space and fits our meager pocketbook. I replaced the DVD player with a real cheap one. They seem to last as long as the ones with all the bells & whistles. I hope to change all my movies over to USB thumb drives in the near future as my movies are over running the bus. And I still have a pile in storage!
BTW, I have since painted the raw plywood black (looks better and seems to disappear). The lower part of the old mantle will have an LP fan forced heater installed and the hearth tiled. We sit so close to the TV, 32" is plenty big enough. I plan on putting a similar or slightly smaller sized one in the back bedroom just for watching movies on.
Go look at TV's and stand as far from them as you would normally be watching them. This will give you a good idea as to what size you will be happy with.