Odds are pretty good you can connect a 2nd charger and nothing terrible will happen. Best case one will dominate and complete a cycle with the other doing little or nothing. Worst case one will get confused by the other and either over or undercharge the batteries. So unless you have a specific plan in mind I would stick to one or the other.
I read that the older converters didn't do a very good job at charging. The first trip in our new camper will be off grid, so I planned on using the generator during part of the day and the battery at night. So I do want the battery changed up each day.
OK, I get it. I would want to test that out at home first before I put my faith in it. Depending on what you have for an outboard charger it may not be any better than the converter. Meanwhile be scoping out a multistage converter as a long term solution.
Go ahead and connect the external charger. The worst case is that the second charger fools the onboard charger into thinking the battery is already charged, meaning its output is much reduced. Or vice versa, the onboard fools the external charger into doing little or nothing.
Are we talking about the converter/charger in that 2001 Viking pop-up? If so, I'm confident it is single stage and has no smarts, letting the battery self-regulate the charge. If you want to take better advantage of the smarter, 3-stage external charger, disconnect one of the battery cables from the onboard system or power-off the onboard charger.
Thanks Gary. Yes, I am talking about the converter in the 2001 Viking pop up. I think disconnecting the battery from the unit while charging with my 3 stage charger will be best. Hopefully there will be no problem with also powering the unit with the generator to run the AC and 120v outlets.
One of the major differences between AC and DC (Battery) systems is this.
With DC you can put a dozen (or more) different "Sources" in parallel
Converter. External charger Battery, Batteries, Solar panels all in parallel and provided that you don't have something like a Magnetek 6300 in the mix (That one tends to overcharge batteries) it works.
With A/C systems you do not put sources in parallel unless they are designed for that. (IE A Pair of Honda EU-2000i inverter generators or the big hydroelectric jobs at Niagria) or they can fall out of sync and major damage (Fire) happens.
But with DC. no need to unhook the on board.
Think of a water system. 3 pumps. each with a check valve, all feeding the same line. The Check Valve is the thing that does the magic. All converters have check valves.