Hi Campswinger. I've had a Waeco 12v/240v unit fitted to my Rienza, and first impressions weren't good. However, I've now discovered how to properly "drive" it! When on the move, I close the vents to the rear, open the front vents and direct them at the driver/passenger seats. It is then fine, and in this recent hot weather, I've often had to turn it down to the lower setting.
My regret was not buying the larger version. I bought the EC1500, so if anyone wants to do a deal on a little-used unit . . . ! It takes a long time to get the motorhome down to the right temperature (a bit like the fridge, really) so get it going before you leave home, whereas a larger unit would cope a lot better.
Of course, the really big advantage is that you can have a cool 'van when you are on site and plugged in to 240v. However, I acknowledge that dash-air is more efficient for the front passengers when on the move. It wouldn't be impossible, though, to rig sections of small diameter trunking (carburettor de-icing trunking?) to take the "coolth" to exactly where you want it.
I'm otherwise very pleased with my choice of roof-air, particularly as we will be spending longer and longer in hot areas in the years to come.