Ahhh, I don't miss living where road salt is used all winter. Sounds like there are very likely more corrosion issues elsewhere in the wiring. Negative wiring usually doesn't corrode, so start by looking at all things that get positive 12v power.
I would disconnect EVERY connector outside the cab and inspect. If it looks good, add some dielectric grease and re-assemble. If suspect, clean as good as you can, replace any really nasty green pins if necessary, and again, dielectric on everything. If you find water inside a sealed connector, blow it out, clean it out, and then use LOTS of dielectric grease. Sometimes just plugging/unplugging several times after adding some dielectric grease will resolve contact issues and prevent them from re-occurring.
If you want not to spend a lot of money, this is something that's easy to do yourself. Just take your time and go through everything. Spray stubborn connectors with WD40 if they don't want to unplug easily.
For the continuous draw, turn the key off, disconnect the negative battery cable and put a decent multimeter in the circuit. Set it to monitor amp draw on a 10A scale. How much draw you see should correspond to how quickly it runs down. A 1A draw will run a normal truck sized battery dead in a few days. 0.25A might take a week. Most new vehicles have ~0.1A of draw all the time, and can't be left to sit unstarted for long periods of time like older vehicles. If you're under .1A, then your problem is either intermittent, and harder to find, or you've got a battery with a partially shorted cell. If there's a draw, start pulling fuses one at a time, checking to see if the amp draw goes away after each one. When it drops to near zero, you at least know what circuit is causing the draw.
If the draw is really low, disconnect the battery and let it sit. If it's dead the next time you hook it up, you need a new battery...
I should also say that if there's any aftermarket accessory that was wired in along the truck's life, you should immediately suspect it as the draw... If you see non-factory wiring at the battery or fuse box, disconnect that first and see if the problem goes away.
And I'll add that batteries do not like to be run to zero volts. If it's gone flat a few times, it's not going to hold much of a charge, and you will need to replace it. If you can't get the draw fixed before you put in a new battery, put in a disconnect at the same time, and disconnect the battery when you're done using the truck. At least that way you won't have to buy a new battery again... I've found that
this style disconnect is cheap, easy to install, and easy to use. I would suggest putting it on the negative side, so you don't have to worry about anything metal touching it and arcing out to ground. Once you solve the problem, you can remove it.
Good Luck!!
