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Here is a great video showing how to test any fuel tank sending unit. Pretty simple, don't worry about the color of the wires in the video, but the process is the same.

The fuel tank sending unit is not to hard to locate. it should be under the seat area. There should be a connector that takes the signal from the sending unit to the gauge. Check to make sure the connector is properly connected first. Then you can measure the resistance from the connector. It would be helpful to have the fuel tank sending unit out of the fuel tank. It is part of the fuel pump assembly. Remove the plastic ring securing it in the fuel tank. Pull it up and out easy, Now you can slowly push up on the fuel tank sending unit and watch the gauge to make sure it responds correctly. If not, then measure the resistance in the wiring. If it does not meet the OEM specifications, then the sending unit is faulty. If it is good, then you will need to check the connections to the gauge. If everything is good at this point, and you are getting the correct signal all the way to the gauge, then the gauge is faulty.

This may or may not be of any use. My 2014 Renegade has the 1100cc engine (same as the trooper). Mine would start and idle just fine. Then one day it wouldn't. It would crank over, fire, then immediately shut down. I found my battery to be a little below 12vdc. I put a battery maintainer on it (basically a 1 amp battery charger to maintain the charge on the battery). SLA batteries have a habit of discharging over time. Mine starts and runs fine. Presuming your charging system is working, measure the battery voltage with the engine off (should be 12.6vdc). You can see if a jump start will allow it to run and idle.