Hey man!
I am fairly new to this whole helping people out on here thing, but let’s give it a shot!
When you open up your GTA V directory, go to the LSPDFR folder, and find the file called “agency.xml”. Once you find it, scroll until you find any of the fire department subsections and make sure where it says something like “<vehicle> firetruk <vehicle>”. Keep in mind that is NOT the specific code for it as I am not at my PC atm, but the important thing is the fire truck model name should be there.
If it isn’t, that’s your issue, if it is, let me know and I’ll go through some other possibilities with you.
Edit: Use OPENIV, not your directory. That’s my bad.