First decide whether you want 'in motion' or not. More expensive, but quicker setup when you stop (or can even use it while moving, as long as the driver can't see the screen). Next see how much height you can use. Raising the height of the motorhome should get serious consideration before doing it. If you already have A/C or the like up there, try to go that height and no further. Of course, you can go up to 13'6" if you have to, but lower is better. These decisions would be your first steps, then you can narrow down between the choices available for these restrictions.
Also, make sure that you have a place for the receiver. Most of the receivers available today won't fit in my entertainment center; fortunately I've picked up several smaller boxes at yard sales and hopefully one of them will do the trick.