for Robbie and Katie's upkeep?

Neither seem to be working while going through school. Robbie is in the guard or something, but that isn't full-time.

There weren't school loans yet so who is feeding them?

In one episode Charley complained about a phone bill, and told Rob that Steve was paying for more than Rob knew.

The only thing we know is that Rob was working a part time job at a drive in. That might have become known in Season 9--I can't remember. In any event, it would have been even worse in that season, after the triplets came along.

I think Steve was helping out.

Rob's hamburger-flipping job was introduced in a Season 9 script.