[Canonman]

I use a battery tender when I remove my trailer batteries for storage. Also use one on the Ram since it sees little use during the off season as well. In both cases I have the batteries connected in parallel. I connect the tender to the positive of one battery and the negative of the other. Electrically this looks like one larger battery to the tender and has worked for me for over 6 years.
For the OP's solar/maintainer idea, I'd suggest disconnecting the batteries from the TT circuit either manually (pulling the positive lead) or with a battery disconnect if equipped. The portable solar panel should connect to a controller. The controller is then connected to the batteries to monitor safe charging levels.