The Univolt in your coach is designed to operate at, say 20-30 amps continuously, day-in, and day-out for weeks on end. If you tried that with your Wal-mart battery charger, it would be toast in a couple of weeks. Also, most Univolts have at least part of the coach fuse panel mounted on them.
Right now, the Univolt in my coach has been plugged in for three weeks cotinuously, charging both the trailer and tow vehicle batteries, running the lights inside, the water pump, fans, etc. 6 amps just wouldn't cut it.
While you don't HAVE to use the exact same unit that came with your coach, a newer (electronically regulated) Univolt will give you better reliability, less power spikes for longer equipment life, and more power (amperage) output than your Wal-mart charger. It will also lessen the tendency to boil the water out of the batteries while plugged in.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.