You won't get any leakage in the underbelly unless you have something else wrong like a bad connection or broken pipe. Our tank indicators have never functioned very well. But we have learned when the shower grey tank is full by watching how fast or slow the water drains from the shower. If we completely fill that grey tank, we will have standing water after a shower. The kitchen grey tank has never been so full that water wouldn't drain from the sink but I suspect that could happen too.
Our black tank indicator doesn't work either, but we can tell the black tank is almost full by the way the toilet flushes (it pushes up an air bubble just as you flush if the tank is pretty full).
I wouldn't waste too many brain cells on trying to solve bad indicators. Most of us don't have reliable indicators either!
Cedar Creek Silverback
Mazda Miata TOAD for sex appeal