If empty or full is high or low resistance matters which way the float is installed. The cool thing is you can figure this out from looking at your SkyView calibration. Go into HARDWARE CALIBRATION then EMS CAL then FUEL LEVEL CAL. In here, you can see what voltage equals what fuel level. So look at what voltage is 0 gallons.
SkyView "pulls up" the voltage and the float pulls it down. So if the 0 gallon voltage is lower than the 2 gallon voltage, then your sensor is reducing in resistance as fuel goes down, and vice versa. In the decreasing resistance mode, then a short to ground would represent zero. In the reverse case, an open wire would represent zero.
As for filters, SkyView supports 3 different "speeds" for fuel level senders. The standard one takes about 16 seconds to change 66%. So if you have 10 gallons, and suddenly go to zero, it will show 3 gallons after 16 seconds.
Don't forget to check your datalogs to see if they have any hints as to what is going on.