I’m pretty sure there’s only one student in a like ability peer group that has a specific academic accommodation, but the accommodation is being applied to the entire group (chunking). It doesn’t bother me that the entire group gets a weekly assignment as part of chunking, but teacher says only the student with the 504 is the one who often comes unprepared not having completed the work before the group meets. My gut say the chunking for the student with the 504 should then be different than the general education experience, but school is disagreeing because they say they are chunking (they could give monthly to do or longer check points)

I agree that the 504 student should probably get something different if the "chunking" is not alleviating the issue (the issue being not completing the work).  But you would need to determine WHY the student is not completing the work before you can determine the appropriate additional accommodation.  

I am pretty sure one reason is because the teacher is not doing the accommodation for “frequent check-in to assess for on task behavior” as the student is frequently left to work independently. 

Under Universal Design for Learning, the same accommodation can be available to the whole class.  Now if UDL isn't working for the student who needs it, then that's a sign that it's not the appropriate accommodation.  Request a meeting so the 504 can be tweaked.  If the 504 isn't being followed, that's a Civil Rights issue and filing a complaint with OCR is the next step if going up the chain of command isn't working.

Ask for data on how often “frequent check-in to assess for on task behavior” are finding your child off task.  Your child might need a less distracting environment or a different sort of help & not more check-ins.