[Richard]

Nope, you are good to go with what you have. The RC-50 will work just fine.

But you have me perplexed because of an earlier question.
If you have a system for monitoring State of Charge (SOC), what was it saying when you were running the batteries down to 11.1 and shutting down the inverter. If the SOC system was working properly, it should have been telling you "Danger, Will Robinson".

Those inverters are HEAVY. If yours is installed like mine. Newell put two studs in the bay wall to hang the inverter from while the four retaining bolts were installed. Those two hanging studs were at a slight angle upward, meaning the inverter had to be lifted up and out to free it. Match the mounting flanges of the old to the new before someone crawls in the bay with it, and holds it up the bay wall for installation. You may have to drill a hole or two in the new inverter to take advantage of the hanging studs, if you have them.

IIRC, Magnum is very specific that the positive cables are installed LAST. That is different than how we normally deal with 12VDC systems. The reason is that many of the control circuits do form a ground loop, and if you install pos first then you could be grounding the system through some very little wires. Not good, and produces copious smoke.