How would you convert a well pump to run off of a truck battery? This well pump takes care of my house and spout near my horse barn.
Most likely you would not unless the engine was running at fast idle and a high capacity alternator was charging the battery. I have an 1800 watt sinewave inverter in my dual battery F-250 diesel truck and it may be able to start and run a small 120 VAC well pump in the 1/3 to 1/2 HP range if it is not too deep, but remember most well pumps are 240 VAC. This would require using two large inverters designed to work together to provide 240 VAC output since most residential inverters are 120 VAC output.
Also keep in mind that the current draw on a 12 volt battery from an inverter will be 10 times the current of any 120 VAC load due to the voltage difference. A 1/2 HP deep-well 120 VAC pump can draw over 20 amps at start-up surge with a run-time load of around 12 amps. This is over 200 amps draw on the battery and DC side of the inverter. Most vehicle batteries cannot take this high current for more than a few seconds, and your battery cables would need to be the size of welding cables, and most inverters this large are 24 volt.
If I were you, a much less costly way to do this is a transfer switch and a plug to connect to a generator.