Hello and welcome to JustAnswer!
What happens when you try to start the engine?
What did you do to check the battery?
The sight glass on the battery is an indication of one cell of the battery, and the battery has 6 cells. Unfortunately those sight glasses aren't very useful at all because they aren't an indication of actual battery condition.
Your problems are the exact ones that come with low battery voltage on one of these trucks. You don't have any electrical accessories that work, the engine won't crank, and the VTSS (factory security) light is staying on. Even on trucks that don't have VTSS it's common for the light to come on when the battery is dead, because the instrument cluster doesn't have enough voltage to properly power itself up. It's either going to be a low battery or a connection problem at the battery, power distribution center, or a ground problem.
If you have some jumper cables you can try them, but jumper cables aren't that reliable. If it does start with a jump then replace the battery.
If it doesn't start with a jump then your next step would be to either swap in a known good battery, or remove the battery that's in it and take it to have it load tested. Almost all parts stores and Walmart will do this for free.
battery was fine last night, will dead batt. keep the security light on?
Yes. Like I mentioned earlier it's normal for the VTSS light to stay on with a dead battery on these trucks, even on trucks that don't even have the VTSS system. The instrument cluster doesn't have enough battery voltage available to it to power up correctly so random things can happen.
Batteries very often fail with no warning. It's not uncommon to have it start fine and then come out of a store and have a dead battery (for example). Things like shorted cells and broken cell connectors in a battery will happen in a fraction of a second, and render the battery totally dead.