If your tanks are exposed and dirty, then when wet you absolutely will get bad readings. Cleaning and coating the nuts/wires with silicone is a great idea (though will make replacing the probes problematic if needed).
There are many causes of bad readings and most are "people" and "environment" caused as you have discovered.
Environment causes include:
Probe location - A "1/3"probe located too close to the bottom of the tank becomes a "not empty" light as one or two flushes can make the connection to the ground probe.
Slimy/dirty tank walls - (IN or OUT) will allow a small trickle of signal to short the probes causing erroneous readings
Levelness of camper - tilting the tank towards the sensors results in tanks that indicate filling faster than they should since the tank bottoms are sloped. In the case of the fresh water tank levelness, not only will the readings be erroneous, they can prevent you from getting a full load of water and/or having a lot of "unusable" water due to vent and pump outlet location.
Bad tank maintenance habits - Using the "Geo" method EVERY dump will keep the inside walls from accumulation conductive slime in the black tank (search for this). My technique if to use a laundry load of laundry soap, CALGON "water softener" (NOT FABRIC SOFTENER or SALT!) with 4-5 gallons of water, after every dump.
15 - 20 minutes if "Turbo-flushing" the black tank when in full hookups every dump; followed by treatment above.
GRAY tank troubles include "longish hair" and food scrapings in drains. ALWAYS use a drain screen in the kitchen sink and shower.
A permanent fix is to replace the OEM button type probe with "Horst Miracle Probes." An easy mod that uses non-stick Teflon stand offs to prevent "cross wall" shorts and bad readings.
Where to buy: