My company has a website that I have been tasked with updating. Currently there are over 100 domains all pointing to about 30 different designs. These 30 designs feed off the same database and code to display advertisements. Each design is specific to a topic or city so the content varies slightly from one design to the next (but not much).

I've just read about how this may be a big no-no with the search engines.

Am I right that these would be considered doorway pages?
Should I use 301 Redirects to point domains to corresponding designs?
Should I create one generic design and redirect all the domains there?

Our biggest competitor has only one domain/website and is doing much better than us. I would like to go that route, but we have traffic coming in from almost all of these domains that we don't want to loose.

Any suggests on how to handle this?

Thanks,
Brad