Do not inter-exchange the batteries. The lower class will plull down th ebetter one and possibly ruin it. Also you will lose "most likely" The positive action of your RV battery. And maybe even ruin it totally.
I would suggest, if you do not boon dock that often, goto a Harbor freigh tools and pick up a couple of the smaller solar panels and hook them in tandem , use them to help your RV battery.
I use panels and most times they hold out very well.
Good luck and do not put these two together. You will be disapointed. Most times if you want dual batteries, it is most wise to get two batteries at the same dealer at the same time of the same size.
That would be to your best advantage..
Well thats all you all.
Chuck