What is usually available by truck from a ready-mix plant is called "flowable fill" but I don't know how thin it can be used.

How are you using the space, and are the walls already in place?

Our 1930s era home had a screened porch with a concrete floor sloped as you describe. When we converted to a 4 season sunroom, we simply glued commercial grade low nap polypro carpet in place. It is technically supermarket entrance carpet. Looks just fine.

I asked a friend who has some contracting experience about this. He had a different approach altogether: scribe cut 2x4s and nail & glue (construction adhesive) them to the concrete floor, then install a typical plywood subfloor. this would raise the floor on the high side about an inch and a half, and on the low side about four and a half inches. It'll be a bit tedious fitting each 2x4, but I think it'll turn out to be the most cost effective solution. Of course, either way of raising the floor means the doors need to be raised up. Since it's getting to be wintertime here in PA I'm thinking this is a springtime project.

Another alternative from a truck is gypsum concrete.

If you want to use wood it is not necessary to scribe cut. Find some straight 2x4's, turn them flat and shim them up at reasonable intervals. You would end up shimming scribed 2x4's to get them level anyway.

"scribe cut 2x4s and nail & glue"

Using 5/4 pressure treated deck boards works very well, and only eats up one inch.