Ice cubes are not going to help you.
AND contrary to the belief of some, sensors DO WORK if you clean up the tanks and sensors and keep them free of debris.
When you are at a CG with full hookups, dump , and then start filling each one of your tanks up to the top with water and dump and repeat and repeat several times.
You need to get all the stuff out that is stuck to the sides/sensors and most likely the bottom of your tanks. Most likely left by the previous owners not dumping correctly. I had to do this with my first used MH and it took numerous fills and dumps to clean them out. Doing it while you are at a CG is the easiest way to do it.
Pick up a couple bottles of lemon cleaner at the dollar store and add to each tank each time you are adding the water to them to flush out the tanks.
I add a bottle of lemon cleaner monthly to the gray and I use chemicals that break down waste and paper in the black. My sensors WORK all the time. :C