RRO spacer is 1.75" right? With stock rims at +56 offset (I think that's right probably want to verify), adding the spacer will effectively make your offset (+56-1.75*25.4) ~ +11.5mm. I am running +25mm offset wheels with 295/55/r20 tires (less offset and wider tires), so you shouldn't have any problem running 285/55/20 tires with the 1.75" spacer. Although, I have read on the forums people claiming that RRO's spacers aren't the highest quality and are prone to misalignment b/c they are not hub-centric. And as far as the Dune Grapplers go, I don't have any experience with these tires so I can't really comment on them other than they do look cool.
This is a good idea, the first set of tires that I bought did not fit: Atturo Trail Blade M/T 33x12.5x20 (too wide, bad rubbing on upper control arm ball joint) so it's nice to have them test out a couple for you to see what you like/ what fits best. But good luck, and upload some pictures when you get it done!
do you think the 33x12.5x20 would have worked if you had a lower offset? you say you were at ~ +25mm but if you went down to say an +18mm offset or lower it would be further out from the control arm?