88 Chevy C3500 7.4L truck has been sitting for 2 years~connected and jumped battery and put gas~drove down the road fine until turned on the lights~truck acted like it wanted to turn off~boyfriend thinks it has a short?

Weak connection between battery post and cable. Since the jumper cables bypass this, by clipping on the cable ends, the car will start with a jump. Takes less current to start a warm car than a cold one, hence it starts all day. I would look closely at the cable ends where they attach to the battery. Clean them until spotless with baking soda and water and terminal cleaner brush.

If this does not solve the problem, clean the chassis ground on the other end of the - battery cable and + cable where it connects to the starter.

Check your alternator output also. Battery should not be defective in 2 years.

Possible current drain when key is off. Put a voltmeter inline with the battery and check for draw. If you get more than a few millivolts something is on and drawing current. Glove box light maybe? Check by pulling fuses until you find the source of the draw.

Have the engine checked for vacuum leaks, that will cause a high idle speed. As a 35 year veteran of auto repair I can tell you that engine flushs are a 100% waste of your hard earned money.

Low oil will not cause the issue you are describing..

need to scan for code autopart store should scan it for free and give you a print out of code.

Most likely the battery. Most auto parts stores will check it for free with the hopes that you will buy your replacement from them.

check both the battery and the alternator if batt. is old or shows weak replace it. it will mess with the computer readings on it

Is the engine light on?

Normally it's just a matter of removing filler tubes from side, disconnect fuel lines & electrical connections at fuel pump/sender, undo the 2 straps, & lower it down.

the 12 amp is for a jump start and the 2 amp is for a trickle charge which doesn't degrade the battery. i believe you have a loose connection either a loose ground cable or the positive cable to the starter check these and tighten if neccessary

need to have the alternator tested sounds like a bad voltage regulator.