Most "Fire trucks" - I'm assuming you mean Engines - carry 500-1000 gallons, usually more on the 500 size. A "Water Tender" generally around 2000 gallons.
> Fire trucks" - I'm assuming you mean Engines
No-no-no, an engine is a part that makes a fire truck move. Fire trucks usually have a diesel engine.
tips fedora
Yes, correct, engineer.
(The generic term for some type of fire-fighting vehicle is Apparatus.)