How do I troubleshoot a water-heater?
You may have already done this but not sure from your question. When testing the thermostats be aware that you must test the input and output sides. A thermostat is really just a switch. First be sure you have 240VAC on the input sides across the hot leads and 120VAC from each leg to ground. Now connect to the load or element side across the element and look for 240VAC as you turn on and off the thermostat. If you have an AMPROBE, confirm current flow when the thermostat is turned to the highest temp position, indicating current flow through the element, and conversely no current when the temp is satisfied. If the voltages are good on both sides of the thermostat, but no current, it is safe to assume the element is bad. If you do not get the changes in voltage at the load side when operating the thermostat min to max etc. the thermostat is bad.
Have you drained the heater? Large amounts of sediment can indicate fouled elements or a partially plugged dip/transfer tube. Be sure to turn off power before draining and refill the tank before turning electricity back on.
 You can also find a toll free number for tech assistance online for all Whirlpool products and should be able to reach a real person in tech assistance that will help walk you through it. not sure if they will do that for the user, but they do that for us contractors all the time.  Good luck.