Domain Mapping and CSS issues

Tearing my hair out trying to figure this out. Using Domain Mapping plugin. The very first site I mapped,, mapped to, forwards correctly but it does not render correctly. HTML is fine, but no CSS and some JS isnt coming thru. When I debug and check the URLs for the CSS and JS, I get 404 errors.

This is a basic setup. No subdomains. Everything is http://www.mainsite/mapped_site.

Any help is appreciated. I'm sure someone has gone through this and I've searched online for 2 days now. Made changes to .htaccess and wp-config to no avail.

Thanks in advance.