To answer the headline, Avatex TV's are expensive (compared to most domestic TV's of the same size) because enough people are prepared to pay more for them, in the belief they are more suited to being used in caravans.
From a manufacturing point of view, they will be more expensive to produce than most sets, becasue they are produced in smaller numbers, and the cost benefits of quantity cannot be gained.
There is also the possibility that a smaller company might not be able to afford to fully develop a technical product to be as reliable as mass producer, but by contrast they might be in a better position to design for a niche market. Swings and roundabouts.
It's almost certain the mass market manufacturers are responding to the buying trends of the public. This has been driven by capability to produce larger screens, which the public seem to have flocked to, (and I'm one of them!). The first television I remember watching was a monochrome 12" 4:3 ratio which gave a viewing area of about 9" x 7".
There are guidelines for distance to TV based on the size of the TV. Bearing in mind the confines of a caravan, going beyond 21" is a little pointless, and also the bigger the TV (of the same technology) the more power it will use.