A couple of suggestions - the indicator lights are notorious for not working well. This isn't a Jayco only issue; most RV'ers will complain of the same problem to one degree or another. In your case, I'd suggest it's compounded by the fact that your trailer doesn't move.
You have a few options for flushing the tank really well, but because the trailer doesn't move agitating the water within the tank will be tough. You won't be able to do it the 'traditional' way (fill the tank 1/2 full, throw in some baking soda or dishsoap and drive down the road for an hour). I'd suggest you hit an RV store and buy a wand that you hook to the hose & stick down the toilet - probably a $40.00 investment to clean the inside of the tank.
As for the leak, I have the panel under the frame of my 23k too; because it wasn't cut to fit really well, water can occasionally travel down the exterior of the trailer and make its way in there. Again, because I travel with my trailer, water will drain out as I drive - in your case, it probably just sits there between the panel and the floor of the trailer until it can find its way out. My thoughts are that your tank is probably not leaking, but you have either condensation or rainwater sitting in there.
IF you're going to go through the headache of taking the panel off, you might as well see about either cleaning the sensors or purchasing an upgraded sensor system for the tanks and install it then. I think they're pretty easy to put in.
Good luck & let us know how you make out!
3 kids and Buddy the Dog;)
2010 JayFeather 23K
05 GMC Envoy XL