A few random thoughts.
If you are going to interface the system to the installed monitor, why not use one of the Arduino's analog I/Os for the lead going to the monitor. Being analog, you can setup the output to be whatever voltage is required by changing values in the Arduino programming rather than having to swap out resistors.
This would make it very easy to calibrate the tank to the monitor by simply changing program variables.
I also got to thinking, a better approach might be to simply rip out the entire monitor circuit. If I recall, the circuit board is simply attached via stand-offs to the backside of the monitor panel.
It would not take much to construct a circuit board to match the one already there, having the same locations for the LEDs and switches, but with an arduino powered monitor. That way, the front-side of the panel would not have to be changed, but the backside would contain a new circuit board with the improved electronics.
But with my next thought, I start to go overboard...
Simply build a new panel. I have used Front Panel Express quite a bit, and a new aluminum panel could be professionally made, and an Arduino could drive a LCD display.
Here is a panel I had made for a camera control system a couple of years ago. I used an Arduino Uno for the processor. The panel was made by Front Panel Express and attaches to a Pelican Case:
It was not cheap - I think the panel was around $100, but it was a fairly complex one.
And finally, I like your idea about being able to see the tank level from the outside of the coach. But expand that idea to include the fresh water tank.
For the project above, I added a RF remote function. Rather than engineering and building a RF circuit, I simply bought one of those made-in-China transmitters/receivers from eBay for a couple of dollars.
I don't see the one I bought, but this one is similar:
http://www.ebay.com/itm/IC2272-4-CH-...item4addd0aea3
But the transmitters I used were circuit boards, not a finished module. But a little searching on eBay and you should be able to find up to a 8 channel (dry contact on/off) transmitter and receiver, or a digitally encoded version - which would require some encoding via Arduino board on the wireless receiver. Either way, it is not difficult.
These cheap little transmitters and receivers really work pretty well.
So I am saying, make a wireless RF tank monitor!!!
And even more crazy, you can buy a bluetooth kit for an arduino board. It would not take much imagination to create an app on an IOS device to monitor the tank levels. If course, we'd have to look into the IOS/Android SDKs, which I have not done.
Problem is, being retired - I am busier now than I was when I worked. I don't know how I ever had the time to work.
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