E-350 diesel hard starting

I work for an ambulance service that has a 1999 Ford E-350 truck. It is a 7.3 V8 Diesel with 36,656 miles on it. Since day one when it sits for more than about 2 weeks it is very hard starting and takes some time for it to finally start. The batteries are all new and maintained, altenators are also functioning properly. It does have a battery maintance system that is also in working condition and the truck is plugged into the shore line at all times when not in use. The truck is also garaged at all times except when on calls. It seems to be a gradual decline in its starting ability. If you take the truck out and drive it, once again it will be starting fine, but then slowly become harder and harder to start. Any ideas?