Can the PV system be tied into the build steel that is up in the crawl space or does it have to be run all the way down to the ground rod?
There must be an equipment grounding conductor run to the inverter. It is not acceptable to use the building steel as a grounding conductor or a grounding electrode. But if the roof is metal, or if any other parts of the building steel are near the open air PV conductors, you will need to ground those metal parts. (If a metal roof, some attachment clamps for standing seams are listed to provide a grounding connection.)
As for the ground rod, the grounding electrode conductor (GEC) that you run to it must be a #8 (or larger, if not run in conduit, see 250.64). What would probably be best is to run that GEC from the grounding electrode terminal on the inverter to the rod, and then tap another #8 to it to run to the AC ground rod at the 100 amp panel, since you must also connect the inverter to the AC grounding electrode system and bond them together.
There is also going to be a combiner box up in the attic. Does the ground from the frame of the pv panels go thru this combiner box?
Yes. And the box must be grounded itself as well.
I had planned on running #10 (black, white and green) from the combiner box to the inverter and the same from the inverter back to the 100amp panel.
Between the inverter and the 100amp panel, the green must be #8, and it must be unspliced or irreversibly spliced. It can be connected (tapped) at one end to the grounding electrode conductor going to the ground rod at the inverter. At the other end it is connected to the grounding electrode conductor going to the rod at the 100 amp panel. It is best to use a c-tap listed for grounding. I'm assuming the 100 amp panel is a main service panel. If it is not, then you will need to go further back to ground to the AC grounding electrode system.
Grounding these PV systems is extremely hard for me to figure out.
I agree. The way I have described is not necessarily the only way to do it.