First, welcome to RVF; we're glad to have you join us.
With regard to the pair of problems you are having, let me first say that I am not familiar with the specific workings of your Damon Challenger. That said, let me offer a couple of general thoughts.
There are two common methods of filling fresh water tanks. Many RVs have a simple filler tube with a cap on it. It might be behind a little door similar to those used for gasoline filler tubes. Water filler doors or caps would usually be on the driver side and in the same area as the water tank. The other common system is a valve plumbed into the fresh water inlet. When you are hooked up to a city connection, you turn the valve to the "fill" position to fill the tank. I'm betting you have one or the other; finding it might be the challenge.
Now, for water heaters, in RVs they come in three flavors; propane only, electric only and dual propane/electric. Let's assume that you have the most common; dual propane/electric.
In the electric mode, there are usually two switches to deal with; one located on the heater itself (inside a removeable grill on the RV exterior) and another inside the coach where other control switches are located. Be sure that the switch (usually a rocker switch) in the water heater compartment is in the "On" position before you try the second switch on the control panel.
In the propane mode, there will be another switch located somewhere near the one for electric operation. When that switch is on, it enables the burner lighting circuit. I think your unit is new enough that the water heater will not have a pilot light, using a piezo-electric spark unit to light the propane. If your water does have the older pilot light system, there should be a metal plate mounted inside the same grill discussed above that will provide lighting instructions.
Without knowing your specific coach, that's about the best I can do.
TJ