I use to sit at the kitchen table, I wouldn't sink in to the seat ha, umm a recliner or even glider, still couldn't sleep in them, I would get about 2 hours of sleep at a time, I did lay on my back propped up legs in Indian style with pillow under them to prop up and bring closer to my belly
I saw a picture of a mom sleeping on a Innertube with a blanket covering it and her belly in the hole. With her neck propped on a pillow. I thought wow, good way to take the pressure off ur back. Haven't needed to try it yet. But I will when I get closer to my due date. Hope that helps.
I had the same thing when I was heavily pregnant with my first. The only place I found relief was in the bath. Back pain (and rib pain) are horrible. It won't last forever though even if it seems like an eternity