Look at a lot of units and write down what you like and don't like- smaller units fit more places. Larger units with slides have more seals to worry about keep a slush fund to fix things. There is always something that will go wrong, heater, ac, fridge, battery tires the list goes on
Good luck you will gets lots of feedback here
I actually did something similar- I had a basic fantastic fan on the overhead bunk ( class c)and decided I wanted to upgrade it- so I bought the upgraded model to install and then put the "guts" of the basic one in the bathroom- its nice setting one to push and one to pull to move fresh air through the rig.
And I don't have a loud fan in the bathroom
For what its worth your city water inlet also has one of these in it - Its a back flow preventer and both failed on my 2008 Winnebago- the hotwater one I replaced the city water I just reassembled.
Found the one in your picture after my winterizing took 2 gallons of water and was bleeding out the drain valve of the hotwater tank
ARB customer service is one of the best in the industry and they have been around for a lot of years. If you can fund the purchase its one of the strongest lockers you'll find also one of the longest in the industry. They are local to where I live and you can call their customer service if you have any questions.