No experience with this, but longer hoses? Perhaps a long ventilation hose connected to both units? I know it would kinda suck when you have to move the trailer - perhaps it could have a screw on clamp to easily remove the vent hose, relocate the trailer and then reattached the vent hose.
What is the layout of the garage? One huge room, or several levels?
Fans? Is their power in the garage to connect to? Create a swamp cooler effect in that you have a fan at one end and then an opening at the other to create a draft that would catch the fumes and carry outside? Would take a large commercial port a cool type unit but you can rent those cheap.
Final option that comes to mind would be a mask/tank set-up similar to what firefighters use.
Lastly, get a portable CO2 detector so you can track levels to know if you are getting close to the danger zone or do what the miners did - get a canary and when it drops dead, get out.