Hi there, this may be of some use.
Along with my Rio I also have a 140 bhp renault traffic van, 2.5ltr.
WhenI went to get some windows fitted, the guy doing the job asked what sort of mpg I was getting - around 35 with a lot of stop starting.
He had the updated version, 150 bhp and was getting around 18/20 mph, had taken it back to Renault several times who could findnothingwrong!
He ended up buying a tuning box (see posts by Davy), and after adjusting for economy was getting 30/35.
Might be worth £100 to try out on your Sedona.
Good luck.