Depending on the converter in the RV you might want to forget a portable, but I'd get a smart charger with a BOOST feature.. That is it should be able to put out 50-100 amps for boost starting a car.. This feature will not be used on your motor home.
As for six volt batteries... Well, yes, I put two six volt batteries in my RV a short time ago, nice new DEKA 230 amp hour jars... However once I hooked up that jumper they magically (Well, it is not really magic, it is simple science, the kind I'm trained in) they became a single big 230 amp hour 12 volt battery.
Think of them that way (As a big 12 volt battery) and watch questions go POOF.
One poster in one of the forums I read replaced just one, and with a smaller six volt..... He ask if this will cause problems.. Let me ask you a question.. Would you want a 12 volt battery where half of it (3 cells) were 230 amp hours and the other half (3 cells) were 190? Right.. neither would I.
This is why your six volt "Twins" need to be identical twins,, Because they are each 1/2 of a 12 volt battery.
Treat them as a single battery. The only exception.. I have had a battery fail within a week of install.. IF that happened Id' not demand replacement of both of them under warranty.