Dave, before I bought and installed my system, I did a fair amount of research and I picked the brains of a lot of folks on this forum. When it comes to inverters/converters/chargers, however, I only know specifics about my system, because I read the manuals, but others here will be able to answer specific questions about your system better than I.
From what I understand, chargers are part of the converter and they're sized for the battery-bank that comes with the RV. If someone were to significantly increase the size of their battery-bank, the charger would still charge the batteries but it would take proportionately longer. And from what I've read, some factory-installed chargers might never be able to fully charge a battery-bank if it's too big. I'm quite sure there's a simple mathematic formula that would tell you how big your charger would need to be to charge whatever size battery-bank you have, but I don't know it.
I think your envisioned 400-500 watt solar array (with controller), would serve your needs well. And yes, other than wiring and a fuse, that's all you'd "need." I would recommend, however, that you also purchase a good battery monitor (not a simple volt-meter). A good battery monitor will let you monitor your battery-bank's condition easily and accurately (I like the Trimetric 2030 for RVs). Pay close attention to wire-size, wire-run length and where your panels are mounted. There are some seemingly small details about solar installations that can have a significant impact on the overall efficiency of the system. When you start shopping for controllers, I highly recommend buying a good quality MPPT controller. They're more expensive but they get more energy from an array that size than a PWM controller.
I'm certainly not the only person here who's done what you're doing, and I'm sure others will give you good advice. Keep asking questions so we can, hopefully, help you avoid some problems and save you some money.
Kev