Maybe someone with a gmc will answer your question. If all else fails, you could measure the space available and then buy the best battery that will fit that space. (I see Dave has already suggested the measure the space method while I was typing)
I did a quick google search but did not find battery specs. I did find a good doc on how the GMC electrical systems work, so I'm including a link here.
gmc motorhome electrical systems descriptionGood luck with your project.
Fred
Fred & Vicki
St. Augustine, Florida