I agree, the locked door is the "gravity feed" point, where you fill the tanks. The one with hose threads bypasses the fresh water tank (but it does fill the hot water tank).
I'll add something to what Rich said about bleach to disinfect the tanks and lines. Make sure you fill the fresh water tank to about 10 gallons BEFORE you add bleach. This will fill the hot water tank too. If you put the mix of bleach and water into an EMPTY system, it will ALL go into the hot water tank, because it fills first (at least on my trailer). Now, run both the hot and cold water to circulate bleach through the entire system (the hot water tank refills from the fresh water tank as long as it is not empty). This is a safety feature to keep you from heating an "empty" hot water tank, which would destroy it.
- Jack