I've never really had a problem with an ER. I call ahead in the evening and tell them I have GP and am in a flare, then ask if they have a free bed for fluids (sometimes my primary care has wanted me to go in and I tell them that). Then I head in and let them know I called earlier. They ask why I need fluids and I let them know I've been vomiting x amount of times for x amount of days in a row, headaches, muscle cramps, dizziness, etc.
I have routinely low BP as well. They usually get the IV set up and then draw blood so I get a bag while they run tests. They usually push IV benadryl and Zofran and come back asking if I'd like a second bag or not. I do not go for pain because I don't take pain medication anyway.
I thankfully now have a script for 2 bags once a month at an infusion center and even though I can't afford to go every month (or 3x a week like cardio wants lol) it's nice to have the order in there for when I'm flaring badly.