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.