New battery -alarm sounds- have to disconnect the battery-

The sounding alarm maybe a security system installed by in the truck. Disconnecting the battery will only temporarily solve your problem, but will resume once you reconnect the battery for electrical power.

You must find the keychain/remote of the security system and disable it. If you are unable to locate this, you can have it disconnected by a qualified automotive electrician.

Word of caution though, if you are using the keyless entry system with the security system, this will be disabled until you install a receiver with such function.

Hope this helps.