More batteries???

I just finished installing the Renogy 400 watt eclipse premium kit, and I bought the 2000 watt inerter/charger in on our 32' 5th wheel. I said I but do to 3 back surgeries in Jan. I had to hire it out. We are the second owners of the RV, we bought 8 months ago. The seller said the two 12 volt MARINE DEEP CYCLE batteries were new. So those were taken out and I bought 4 interstate 6v golf cart batteries 120AH each and that is my new system. These two 12 volt bat. that I took out have been sitting in the back of my pick up for close too two months, yesterday I put a volt meter on both and they were 12.7 each. So it got me thinking, I know you shouldn't mix match batteries but what if I made another systym? I may not even need it, just thinking of a way to use these two 12 volt Bat. I still have to make a few more 12 volt hook ups in our bedroom, C-pap, a couple of charging stations also want to install at least one fan in the basement, all of which I think my 6 volt bat. set up would probaly work why not lighten the load with the two 12 volt. Batteries and maybe a suitcase system two charge the other two 12 volt bat. Or should I leave well enough alone and wait.
Thanks, Mel