We also have an A214HW. The water filter and pump are under the bed on the driver's side. You will find a plywood cover held over them with two screws. I recommend removing the screws and tossing them since the plywood can't really go anywhere. Makes it easier to maintain the filter.
The switch is under the water heater switch on the box under the front bench seat. You can't miss them. The power converter is on the outside and the switches are where your legs go under the table.
You can use an Aquatainer on the shower side just like you do on the sink side. Using a hand shower actually doesn't use that much water so a 7 gallon is probably enough per shower. We actually don't use our inside shower because we learned a long time ago that once you take a hot shower in the trailer you spend all day with the A/C (or heat pump in our case) trying to get the humidity out
This is our fourth RV in approximately 30 years. And my DW refuses to boondock so we always stay where there are facilities.