From the SEO standpoint google will likely detect the identical content and will only use one of the domains while sandboxing the other. If you want to force the use of one domain over the other while allowing both domains to resolve to the same site, do this:

Assuming you want classcampers.com to be your main domain:

1)     Point both domains to the same name servers, also make sure both of them are aliased to the same IIS site. Depending on your providers control panel this function may be called “domain alias” or something else.

2)     In DNN make sure only classcampers.com is defined as portal alias, leave classcampers.co.uk unmapped to any portal(s).

If someone uses classcampers.co.uk to access your site, DNN will transparently redirect to classcampers.com domain. When it comes to SEO there is a lot of Voodoo science, however what matters is good text based content, age of the domain, and links from other high ranking sites.