Gary,
When I spec'd out my 2552 in late 2018, the See Level II system was a $495 option on the Phoenix build sheet. Although I had zero RV experience, based on everything I had read on the forums about the unreliability and shortfall of the standard "light system" monitors I decided to add them to my build.
To date, I am completely happy and have mentioned in a couple of previous posts that I really like the performance. You get a 1% change on the readout for each monitor (fresh, grey, and black water, and LPG), and 1/10th volt readout on the battery. Based on my use, they have been accurate, and you develop a sense of confidence that you know the exact status of each of the tanks.
Mike (Gandalf) asked me once before about how they were installed on the foil-wrapped black and grey tanks--and I don't know if we ever followed-up with the exact answer. On those two tanks, Phoenix trimmed the foil wrapping exactly on each side of the sensor/sender boards, and the sensors are applied directly to the tanks.
I see from your link that the listed price for the system is $200; I assume that Phoenix had some mark-up on their unit and the balance of their price difference was installation labor. Based on my experience so far, I would consider the $200 well-spent. That could change if the reliability of the system doesn't continue for some reason. So far, so good.
Mike