Hi we have just had a Fiamma Turbo Vent with Polar control fitted to our Elnagh. We hope this will help keep us cool in the years to come.It's thermostatically controlled or manual & certainly seems to wizz round at a fair rate & in either direction . it needs a 40cmx 39cm opening ,also has a low consumption rate.
Here is a link to the Fiamma website direct to the fan.
http://www.fiamma.com/EN/products/quality_motorhome_products/vent/turbo-vent/

The cost including fitting was 270 euros ,got a 10% discount at Cruz in Elche Spain.
Tech Detail- Input Voltage 12v +/- 30%
Absorbed power - 36W
Min input current - 0.3 amps+/- 10%
Max input current - 3A +/- 10%

Will let you know how it performs in the very hot weather . Bye all Bill & Pat