If you can afford, think about getting a 5# ABC ($30-50) and a 5# CO2 ($200 approx for 5# CO2).
I'd prefer to use the CO2 if the fire was in the engine area or if there are sleds the trailer and a fire in the trailer.
Once you use an ABC, even slightly, the pressure will leak out (probably within the next month) and you will need to get it recharged. If you use the ABC in the engine area, it will make a huge mess and I heard the chemical is corrosive. I'd be concerned about the clutches, intake, carbs, etc, being coated in the chemical.
If a quick shot of CO2 puts it out, and you still have the 5# abc as backup, I wouldn't worry about recharging the CO2 because they generally don't leak after use because there is no dry chemical to get in the valve mechanism.