Some of the tank monitors are hall-effect, and some are simply resistive. They are just a gauge with a sensor attached to the dial.
I am sure the resistive version is a lot cheaper, and for that reason, it's the one I'd guess most common in the RV industry.
Remember... the RV industry isn't anything if not cheap!
Similar or identical to this one:
http://www.amazon.com/dp/B003D3WPGS/...29c02f6c912d_S
The ones I found are $30~50 for a gauge, which is too expensive for testing.
I typically document my toys (RV, boat, etc) for future reference as well as each time I work on it. I found a photo of the gauge on my rig, and it looks identical to the one at Amazon. I will confirm that when I can get the rig out of the snow bank. In the mean time, I did ask the company that makes my gauge for a spec sheet. Have not yet heard back from them.
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