I need to point out the advantage of the OP's idea. Yes, it is inefficient, but efficiency isn't everything. The battery doesn't care what was lost upstream, it cares about the voltage applied to its terminals. The typical vehicle charging system does a pretty good job of keeping the nearby starting battery charged, but getting a deep cycle battery back to full charge some distance away is another story. The converter in a '99 camper might not do much better, but modern converters monitor the voltage and use multiple charging levels to get the battery back to full quickly.