It seems you’ve answered your own question, By saying the water doesn’t seem to get hot..would explain the lack of color change.
It is simply a diverter...capable of returning water to the fresh tank, while you purge the lines of cold water. This...with or without a color change will save a bit on water usage.
If the water isn’t hot...the source has nothing to do with the aquamiser. It is the hot water heater/Oasis...or whatever you have. I will say...replacing the standard shower valve with a better, pressure compensating type will give you improved results. We had horrible luck trying to get consistent temperature in our Ventana until we replaced the valve with a Moen Alder fixture. Now, it stays where we set it.
I would start with making sure the system has good hot water. Then if you want, open the diverter all the way...open the shower valve to full hot...use the color change, or just wait a minute or so...readjust the valve to the location you usually find comfortable...then flip the diverter.
If it weren’t such a hassle...I would consider adding it to our 2016...but we will just have to sacrifice some water each time we bathe...
I think the Aquamiser is a simplistic but somewhat functional system. If you want to go all out (semi nonsensical) you might get this system retrofitted into your coach...