I'm so sorry about little Evie, Purple. Fudge and I send cuddles and well wishes.
I went through similar with one of my mice last year, except she never made it to the vets but instead passed away half way there and I was heartbroken as I'd nursed her through several bouts of RI and really thought she was getting better. It's never easy to face the prospect of losing a lil fur-baby no matter how small they are.
One of my facebook friends said to me that it's amazing how such a tiny animal can leave such big pawprints on your heart and it's true.
You've given Evie the best life possible and she knows that.
Just keep giving her cuddles and try and get fluids in to her. You never know, there's still time for her to perk up and Gerbils are hardy little critters
One thing you could do to keep her warm is to wrap a hot water bottle in a towel and lay her on it if she can't keep her temperature up herself.
I really hope she pulls through, it's obvious how much she means to you and how much you love her.
Keep us updated xx