I think that you access most of that stuff from the inside of the cab. My '02 Journey has a one piece front end cap so I don't think that's how you access that motor. It's inside under the dash anyway so it would make sense that it's mounted before the dash is put on.
This is a diesel pusher right? If so, there will be two water shutoff gate valves on the engine in two places, close to the water pump. Follow the water hoses from the front back to the engine to find them. My Cummins valves were both accessible from underneath the engine. I think my Cat has them top and bottom. Shutting those off will stop any water flow from the engine while you're working on the heater & A/C system. Also note that there is an air valve on the outside of the firewall on the passengers side that is controlled by air. The rubber hose that actuates it often rots and the hose splits. You can't see it without craning your head around so you can see the back of the air valve.