Does your Magnum have the "auto" setting like the Xantrex? If not, I think I would just choose the 800 for now and then call magnum and ask. I think my xantrex 3000 says to choose 1000 for my 8- 6v house trojans. I also believe I read that it takes a few cycles for the "auto" setting to "dial in". Not sure how the chassis 12v's fit into the equation but sounds like a non-factor in the xantrex book. Let us know what you find out.
On edit: did you also install the battery temperature sensor with the new Magnum or could you use the old xantrex set up?
2016 Arctic Fox 25Y (For Sale), 2016 F350 4x4 DRW, 6.7
2008 Mobile Suites RE3: Sold
2005 Monaco Dynasty Diamond IV: Sold