Your Tundra should have a Starter Relay, and it's likely in the main fuse block under the hood. I checked your Owners Manual but it doesn't look like they put the information in there. Fuse Locations are likely molded on the underside of the Main Fuse Block Cover.
As @SolarMike said, you should remove, clean, and tighten grounds and battery terminals. Check for signs of creature nesting, and carefully inspect wiring if anything is found.
YouTube has a video on testing a Toyota Starter Relay with a Multi-meter. While you have a Multi-meter, you should also test to be sure that your battery is charged (Above ~12.6V) You could also remove the battery from your truck, and bring it to a Retail Auto-parts store for free testing if you don't have a Multi-meter.
I'd highly suggest investing in one, though. They are very, very, helpful for numerous Automotive, and other, projects.
Disconnect the battery, and clean and tighten the Ignition Wire going to the starter.
If everything has checked out from there... The next step would likely be a Voltage Drop Test across the wires involving the Starter Circuit. Here is an explanation of how to perform a Voltage Drop Test and a different explanation here.
I hope this helps!