Your fiver is being charged by your alternator. However, the wire run from that alternator to the battery is VERY long. So unless that wire is fairly large say at least #2 or larger you are not going to get enough voltage to the batteries to charge them efficiently. They will charge. It will take a long time to get them fully charged. Back to the large wire: the natural limiting factor is going to be the seven pin pigtail from your truck bumper to the rig. That pigtail has all the wiring to run the brakes and lights and the charging. But that wire is definitely not #2 or larger. Probably something like #10 at the most.
Bottom line: if you are planning on boondocking it is unlikely you will be able to keep your batteries charged using your truck. In my signature block there is a URL to my blog. One of my entries there discusses my own experience with this exact problem: expecting the truck to keep my batteries charged between camping spots.
You will also see that I am a big solar proponent on RVs.
Enjoy your new rig!
Show Low and Rio Verde, AZ