Ha! I know where Brackettville is! I drove through there quite a few times when I was stationed at Laughlin AFB in Del Rio on trips to San Antonio. That was back in the 60s. I bet I wouldn't recognize anything now.
Welcome to the HiLo family!
Regarding your tank monitors, I believe there is a replacement brand that uses slightly different probes (you can just replace the probes if the internal panel is ok) and these probes are designed to resist false readings a lot better. It's what I'd use if mine went bad. Do an internet search for "RV Holding Tank Probes" and I think you'll find them.
Edit: To save you time, I found them: http://www.rvprobes.com/howitworks.html
But, there's also a completely different kind of sensor that's basically a strip that you run vertically on the outside of each tank. The panel inside then gives you a true percentage fill reading of each tank. It looks to be a bit of hard work to install to me, but you could also use that.