If the coolant reservoir magically empties itself, you definitely have a leak.
How old is your truck? How many miles?
Do you see any signs of a leak under the truck? Toyota OEM coolant is red, sort of like trans fluid but a little thinner. You may not see a leak... I've had two Toyota's that in the past had a coolant leak, but it was so smalll that the warm engine block evaporated the coolant before it got to the ground. It wasn't until the car sat for a couple of weeks that I found a small red puddle. One problem was a freeze plug, and the other was the water pump bearing. Pull off the covers underneath the truck and look for signs of leaking fluid that might be drying up.
Do you have white smoke coming out of the exhaust constantly? (Not just water vapor in cold air, but actual smoke.) If so, that could indicate a head gasket leak. That's a bigger deal, because then the coolant is mixing with the engine oil and potentially causing other problems.
You should always try to keep the reservoir full. Keep an eye on it, and if it gets below the bottom line, fill it to the top line. Now that you've been driving a lot with low coolant, you'll also find that when you fill the reservoir it will disappear a lot faster as the system makes up for the amount it has lost. Just keep filling it as needed, and eventually it will stop consuming coolant as quickly. Then you can focus on finding the leak.
