No, backfeeding into the converter is fine - if you think about it, that would be happening every time you're running off batteries. In particular, in my motorhome, when the engine is running, it powers the house 12-volt system, and that's no problem for the converter either. (Or, at least, it shouldn't be.)
You shouldn't need to do anything special, just a fuse like 68 Overlander said.
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab