I looked at a couple YouTube dealer videos for your Outback model. I have the Outback 298RE, there are many similarities. Here's what I think. The water pump is on the Driver Side of the trailer beside the bed under that box. Mine is in the same location. The water heater is forward of that.
When you look in the pass-through under the bed, you can see the framed in boxed area that surrounds the water heater. My water heater is in my kitchen area.
I can see the 3 screws you mentioned, and it looks like they just hold on trim. I think to access the water heater from the other side, you have to get to it from under the bed.
Raise the bed, and see if there is a panel on the side of the bed frame that is removable.
By the way, your water heater will have only 1 valve. It uses in line check valves when in bypass position. You will NOT have 3 valves.
Look under the bed beside the water heater. See if the access is there.
By the way, that's a nice trailer. I love the Outbacks. I'm not prejudiced in any way ... nah! not one bit!