Not changing my answer (yet), but if you want to get dizzy, follow the artillery reserve routes taken by day as documented in the book link below. I am questioning my sanity because I have consumed so much time on a beautiful holiday weekend researching this question. I have attempted to follow the routes taken by the artillery reserve using different state maps. I can safely say there is not a consensus on anything especially what Corps they marched with. While I am sticking to my original answer as it is all that I have, it makes no sense based on other sources. Hopefully one of my Gettysburg books discusses this. Rant over, hopefully everyone had a great Memorial Day weekend. Rant over. Serenity now, serenity now.
Last post, Coddington does refer to the 2nd corps traveling behind the artillery reserve. He infers also that the 2nd and 12th Corps were traveling together. So maybe the 2nd & 12th Corps are correct. Thats all I could find. My apologies for 3 posts, but almost 16 hours of searching has made me insane.
Source: The 12th Corps and the 2nd Corps and the Artillery Reserve would march toward Taneytown to Middleburg, and Woodsboro, through Frederick to Middletown.
SOURCE: The Life and Letters of George Gordon Meade
Edit - It is apparent that various sources give numerous different answers to this question. I note in particular that the source cited in post # 22 to support the answer of 6th and 11th Corps is the same source that was cited in post # 26 to support the answer of 2nd and 12th Corps.
I conclude that the Artillery Reserve traveled with different corps at different times. In order to rule anybody's answer incorrect, I would have to try to find sources that would prove that it didn't travel with a particular corps at any time during the Confederate retreat. That would be more of a task than I'm willing to undertake.
Everyone who submitted an answer will get credit for a correct response to this question.