I'm in the exact same boat as you are. RRT who became a RN, stayed at the same hospital, now working in SICU. I leave pretty much all of the RT work to my fellow RT's (Vent changes, ABG's, retape ETT's, breathing tx's, extubations, etc.). I have more than enough work to do as a nurse.
However....I will not hestitate to make changes in an emergency and I do try to keep current on ABG sticks by just asking the RT if they mind if I did the stick, when I have time. I have never had a RT tell me "No, I don't want you to do my work for me!" Of course, I always talk to the RT if I make any changes. I have never gotten any flack from them. Of course, I don't know what they say about me behind my back! HA!
Waiting 10-15 minutes for a RT to show up for a vent change/abg is unacceptable in my book and if you have to wait that long, I think you should have the right to make changes/do abg's as a RT. Why does it take so long to get a RT to the bedside? If you can come to an agreement with your manager and the RT department about your duties, that would be good but in the end, I think it may create more work for you because the RT will say "Oh, that's Lisa's patient, she can do all the RT stuff!" and it may cause some resentment on the part of the other RT's. You know how territorial RT's can be!
I already find myself being used as a RT resource on the unit by the other nurses when the RT is not around and they will ask me to check their vents/explain why changes were made after the RT has been there. I don't mind that at all. Helps keep me thinking about RT stuff and I don't hesitate to bring up the waveforms on the vent and make adjustments to Esens, PF, flow trigger, etc. if needed to make my patient more comfortable.
Good luck with trying to get those duties in writing. I've seen you posting on the CRNA pages. I am interested in that career path also. If you'd like to compare notes, I'd be happy to keep in touch. Take care.