You can't use an "older dumb charger" (also called a "manual" charger), because it just keeps raising the battery voltage higher and higher past what is proper into the 16v+ range if you forget to turn it off.
The issue here is with modern chargers that are "automatic" (not "manual") that shut down when the battery is charged, as opposed to modern ones that go into a "float" mode after the battery is (sort of) full, so it keeps supplying 13.x volts like a converter does.
Note the Xantrex Truecharge type of three-stage charger which lets you turn off the third stage so it does shut right off when the battery is sort of full. You can choose to keep the third stage working.
Some chargers have no actual float stage, but do stay on and come back to life after the battery voltage falls to a certain level, do a recharge, then go back to sleep. No use as a substitute converter, but good for other scenarios.
Best bet to cover all bases, is get a second deck-mount converter and use that as your portable charger. Cut off the end of a set of jumper cables with the clamps, and stick that in the converter's DC output terminals. Clamp on and plug in, same as any portable charger---but now it stays on like a converter, because it is a converter.
