A class cab heater

Hi all,

I have a 2004 Burstner A-class built on a Fiat/ ALKO chassis. The cab heater is little more than useless; it blows lukewarm at best on a cold day. My supplying dealer has had it in twice now but with no real improvement. They say that because the front end is much wider than the van from which it was derived, that the heating systems struggle, and that it is a common issue on older A-class vans. Has anyone any experience of this problem? Any input is greatly appreciated.