Won't crank sometimes?

Truck has been cranking and running with very little. Problems. Had been using my truck to move all day, then stopped to fill up with gas. Just as we were approaching our apartment; the truck made a woosh sound as if it was clearing out a valve. Then the truck started to slow down so I pushed the gas harder and it made another woody then sped up. Since we have been home it will crank when the engine is cold but will not stay started. When the key is turned it bring up a battery symbol and needs oil message. It's a fairly new battery, but when I checked inside it the fluids were low so I added water to them. All the lights, windows are working with the key tuned in the ignition. Even the humming from the gas pump I was told to listen for. The only thing different is the check engine light that's been on went off the night before this happened.

I have owned my truck for 2 years and it has always started very slow as if the battery was not powerful enough. Yesterday was a very cold day in Ohio and my truck finally did not start. I took the old battery out had it tested and indeed it was bad but was the correct size battery for my Avalanche. I replaced the battery with a brand new optimus battery with 730 CA which cost me $160 dollars. Even after the new battery is starts very slow. This morning I had enough power to unlock my doors but not to start. I tried to jump my truck but did not have any luck. I put the brand new battery on a charger and it was down to 75% charge. Do you think I have an electrical problem or a starter problem?

It took a week for you to tell me to check for corrosion. Nice website. Well, a week ago I changed my starter/solenoid and that was the problem. The solenoid had a slight crack in the casing. Starter cost me $112.00 Thanks for all the help, I'm sure I will return next time.