I have reviewed several threads on this topic all of which ended without a solution being presented.
I have Infinite SEO and Domain Mapping installed (both your plugins and the latest updated version).
With Domain Mapping I have selected to use Mapped Domain in the Network Options.
The sitemap that is generated by Infinite SEO uses the subdomain (unmapped domain name) rather than the mapped domain name.
I read a few threads that suggested that the sitemap may have been created by accessing it from the sub-domain which doesn't really make sense if I have selected the use Mapped Domain option in the Domain Mapping plugin Network Admin page. However, to test this I deleted the file that was generated and re-generated it but to no avail - I still get the sub-domain.
I think this is a bug in the Infinite SEO plugin because on the Sitemap tab it displays the url for the site map using the sub-domain. I think this component of the coding is where the bug is.
If a solution has been identified for this issue and you can point me to the correct thread I would appreciate it as I have read more than a dozen threads that relate to this issue and none appear to be resolved.