This is a the problem with units that are seasonal, they dont' move and if any TP or other substance lands on the probe its pretty much there for the duration.
Its probably to late for the softner and detergent to do anything right now, as for the ice good in thought but it will probably melt before you get your rig off the driveway.
My thought would be to fill your tank about a 1/4 or 1/3 and get into some stop and go traffic. Get the water to slosh around violently. Sometimes you get it first attempt sometimes not.
Those probes are notorius for being inaccurate, they are more so a guide line.
Maybe when you tank is actually full the indicator will register as full. Thats all you pretty much need anyway. Once the indicator says full you have some flushes left but you will need to dump soon.
My understanding for the water softner is this will soften the water and materials are less likey to stick to the tank walls and probe. As for the detergent, well that just cleans and helps dimish the odour.
Well thats my couple of pennies, I'm sure others will pipe in and may be able to help you better.
Enjoy the time out and don't fret of this........