I have worked in an ER for almost 7 years, with the last one spent traveling. I have also worked as the clinical educator for an emergency department which saw 80,000 patients a year with a staff of 200. I am not sure if an internship where you are is the same type of internship we had at our hospital. An internship is typically designed for nurses with less than 6 months of experience, not for seasoned nurses.
Based on your post, it sounds as if you already have plenty of experience under your belt and would most likely not need a complete internship. And like the previous poster stated, you've already started mastering the hard part of patient care in the ER: juggling your priorities. In your case, an internship may make you feel as if you have had your wings clipped and are going back to square one. What may work best for you would be a hybrid. I would start orientation in the ER with a preceptor, but also try to take many of the internship classes that are offered.
Many times the internship classes will go over some of the finer points of ER medicine that you may not be exposed to anywhere else in the hospital. For instance, do you know your PALS and ACLS inside and out without the algorithms? Or are you familiar with the initial management of an ingestion of various drugs (rx and illegal) upon immediate presentation, as well as possible unknown substances? What are your immediate priorities when caring for a burn patient? I am not throwing these out as a means of intimidation, so please do not misunderstand me when I say that (have you seen an ER nurse try to deal with a PCA pump or set-up and maintain an art line??? - we have had floor nurses & ICU nurses come down to laugh at us about it. So, we absolutely have a mountain of things that we have no idea about). What I am getting to is that there is so much of what we do as ER nurses that is second nature to us that we sometimes don't articulate it as well when we are precepting. Those are the types of things that an intern class would probably help illuminate for you.
So many full blown internships will place you in a process which can take anywhere from 6 months to 18 months and is usually a pre-set laid out schedule with little room for alterations. The reason that the ER has created internships was because the ER is a sort of "baptism by fire" and a bit like the Army, "just do what I tell you to do when I tell you to do it." What we found out was that there was a high turn over and burn out rate because so many felt that they didn't really learn to be a nurse before getting thrown into an environment that expects you to be a nurse, an on your toes critical thinker, and have the ability to constantly reprioritize your patients and their care. And oh yeah, you're expected to be all that from day one. Internships were created so that less experienced nurses could learn what it is like to be a nurse first and then progress into the demands of ER nursing. Based on what information has been filtering back and has been published, an internship in the ER helped decrease some of the turn over and burn out. It also gave the new nurse more confidence in the care they were providing because they felt that they had an opportunity to learn the aspects of patient care in a slower pace (in an environment that is anything but).
ER nursing is a lot like the military "It's the toughest job you'll ever love." You already have the most difficult part of ER nursing down, you know how to take care of patients and manage multiple components of their care at the same time. Therefore, I do not feel that a full blown internship would be of great benefit for you. I would definitely start the unit orienting with a preceptor and try to take intern classes when appropriate. The best part of that model is that it allows an adjustment for time of orientation based on YOUR needs, not according to a pre-set schedule set up by someone else. I say take a deep breath, jump in with both feet, and enjoy the ride! Just have faith in yourself and everything else will work out according to plan.