Hi there. SEA is an Italian firm that used to own the Auto-Sleepers Group here in the UK. As a result, a number of SEA brands were imported here in the mid noughties, but not for the last four years since Auto-Sleepers Group underwent a management buyout.
Times have been tough for the motorhome market in Italy, and SEA was acquired by the French Trigano Group in December 2012. Trigano also owns Chausson, Auto-Trail, Roller Team and many more.
Here's a bit more info about the Trigano buyout:
http://www.practicalmotorhome.com/news/trigano-group-buys-sea
To my knowledge, SEA never owned Adria - the latter is a Slovenian company and has been so for some 50 years now.
We tested a few models down the years and found them to be basic but good motorhomes. The only one review I can find here on the website was about a mobility-special model, but it'll give you a bit more info about SEA:
http://www.practicalmotorhome.com/review/motorhome/2006-sea-cpt