Hi rch and welcome to the groups!
The problem you're most likely running into is that Google grabs
Sitemap files using the normal Googlebot. According to your robots.txt
file however, you're blocking all accesses by the Googlebot, so we
can't grab your Sitemap file.
A simple solution would be to just allow the Sitemap URL, for
instance:
User-agent: Googlebot
Disallow: /
Allow: /sitemapfile.xml
In this case, the more specific entry is the one we follow, so we
would have no problem accessing http://domain.com/sitemapfile.xml
using a Googlebot.
Hope it helps!
John