Not all trucks are fitted with an LPS switch, just trucks that were purchased with the "LPS" kit of added safety options. If your truck has one, when new, the truck had stickers on the sides that identified it as an LPS truck, and the switch would be somewhere in the heavy cable negative circuit between the battery and the frame, and would have a method of opening the circuit that did not require the hood to be opened to engage the switch to open the circuit.
If your battery is charging 14.2 at the battery post (not neg on the frame of the truck), with all the lights and accessory on, then it is NOT likely that the charge circuit is the problem. if it changes voltage between the frame and the battery post, then the ground may be where the problem is.
the codes E24, E31, E35, E36; break down as;
* E24, is the accelerator pedal circuit sensor malfunction
E31, is the ETC (electronic throttle control motor) circuit.
E35, is the LPG injector circuit malfunction.
E36 is the LPG fuel pressure sensor malfunction.
What is the possibility that you have gotten moisture and dirt into the connectors of the ECM?
Do you have access to a 'test' ecm to test the ecm by swapping it temporary?
I have seen some ECM where the pins to the connectors got pushed back into the ECM due to debris or mis-fitting the connector.
* because the code e24 is related to the accel pedal circuit, you are going to have to have someone with the program and laptop to come out to clear the codes as a code for the Accel pedal can not be cleared using the accel pedal "dance".