Battery replacement. Standard 12volt or upgrade?

Should not affect the warranty. I would not go to the expense of twin six volts based on usage you described. I can tell you that our family uses twin group 27 deep cycles and we can dry camp for three nights watching video's every night with very little furnace. After that we charge with Honda generator. If only unplugged for a night here and there one good deepcycle 12 volt sould be fine.