Mine puts out plenty of heat, even when driving in 30 degree temps recently over Donner Summit at night. I had to put the fan on 3rd from low setting (mine has 4 steps) for plenty of hot air, but it kept me warm enough to drive w/out any other heat.
Check control valve function, check heater core box to see it something built a nest in there, check heater core for plugged condition, check damper to see its closing (not 50% outside cold air straight thru), etc.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project