I think you mean converter. But in any case it is a bad idea to hook up to 120V AC without a battery in the 12V system. Voltages can jump around without the battery to stabilize them, but most 12V appliances are designed to run on anything from 11 to 14 V.
If you have electric brakes, you will have no breakaway braking.
Just go down to your friendly Costco, Sams, WalMart and buy one. If you never use it alone without 120V power, then the model doesn't matter. Look for the cheapest that will fit.
David