What to fill this in with?

[gstrom99]

gravel

What I meant by "gravel" is "road gravel" or 3/4 or "with fines/screenings". Whatever your local quarry guys call it there. Pure 1" "loose" is not what I meant...

 

[Engineer42]

Figuring that you are intending to clean up the debris left on top, you should smooth the exposed surface as best you can with your tractor. Then place crusher run or crushed concrete between the slabs. Track it in with the tires, make sure it is as level as the slabs when done.

Worse case it that's too much money, it looks like you have room to swing the Rv around and drive it on the slabs going forward. Have your wife guide you, and follow her directions. Just ramp up to the high end of the slab to save your tires.

 

[rScotty]

You '"red dirt" sounds like clay. If so, the reason those strips are still level has to a lot to do with decent drainage and maybe the double wide was spreading the load & keeping the area dry.
You could try pitting some small as you can get type of crusher run down between the slabs. buy the crusher With the fines. Place it in small one inch lifts compacted with your tractor tires while slightly moist until level. At least that will make it look better. With luck it might even keep the clay stable. Put some thought into perimeter drainage when done. All that wouldn't cost much and might even work.
I would NOT just try to fill in with concrete. If you want to go to that much trouble, a better way would be to prepare the ground, place some steel, key to the slabs, and then pour a sab right over the top of the existing.

 

[EddieWalker]

I'd spend the money and pour concrete. Nothing will be nicer then a big flat, smooth concrete pad. But then after the concrete is poured, I'd build walls and put a roof over it!!!!