Question for those (much) smarter than I
 

Back in December, I thought my battery was dead since the truck wouldn't start even after a charge. Got the new battery in and everything worked great.

Fast-forward to this weekend...truck is doing the same thing. Starter motor will make one feeble 1/2 attempt and then nothing. Lights won't come on either.

I know these old trucks aren't that complicated. What and in what order should I troubleshoot?

Re: Question for those (much) smarter than I
 

how often do you drive it? if not very often, it may just be dying from non-use. it could be a parasitic drain, like a switch you didnt know you had on (the dome light?) or from a bad key switch or bad winding in the starter solenoid. use a VOM to find out if something is dragging current when eveything is off, or disconnect the battery when you park it for long periods.

generator or alternator? generators need the brushes replaced once in a blue moon, if you are starting it and nothing is charging or it is charging poorly, it could be the belt is loose or the generator needs service. if alternator, the voltage regulator could be bad, use a VOM to make sure you are seeing more than battery voltage when the truck is running.

could be your cables too, look for that white corrosion at the ring terminals, or check for a loose or poor ground to the frame or body or engine.

Re: Question for those (much) smarter than I
 

Sounds like something may be putting a draw on the battery when everything is off.
An easy way to test is to series a load on the battery positive cable. An automotive lamp makes a good load. I rigged up a load tester with an 1156 bulb and 2 pigtails on an aftermarket lamp socket, one is the "hot" and the other the ground.
Remove the pos cable from the battery and wrap one of the pigtails of the tester to it. Wrap the other pigtail on the battery pos terminal, I use a pr of vice grips to hold it on.
With the key off, if the lamp glows you have something draining the battery. You can remove one fuse at a time to isolate. Some circuits like the charging circuit isn't fused (if your truck is original) so you'll have to disconnect wires one at a time.
If it doesn't glow then you don't have a drain and you may either keep a trickle charger on the battery and/or disconnect it when you let it sit extended times.
Good luck

Re: Question for those (much) smarter than I
 

I'd go along with the parasitic drain concept. Put the charger on it and fully charge the battery and get the multi-meter out and start checking. I've seen several that had a drain across the top of the battery due to dirt and moisture. You can check that with a fully charge battery by putting the + wire of the meter on the + post and touching the - lead to spots around the top of the battery case. There should be no reading and any reading is your drain. Also check and clean the cable ends as they may have corroded over time.

Re: Question for those (much) smarter than I
 

Fixed it. Appears it was merely a loose ground connection at the battery. Charged the battery, tightened the posts...started right up. Glad it wasn't anything more complicated than that. Thanks to all who replied!