Two RV/marine batteries and a single 100 watt solar will for a frugal camper with a gas elec fridge give you several days of boondocking with a TT.
Adding a second solar panel would give the most added bang for the buck because it will even be charging the batteries while you are moving.
You don't mention equipment but likely you would need a new charge controller to accommodate the increased power, if so getting one which will accept higher input voltage (36 volts) should allow you to wire the new panel in series and then avoid having to run new wire from the panels to the controller.
Adding an inverter (if you don't already have one) will let you run some limited 120 volt ac items, tv, small appliances etc. Size is dependent on what you want to run.
Also as a back up to charge the trailer batteries in a pinch, you can use your tow vehicle. Simply positioning it to let you hook a set of jumper cables to the truck while idling for a half hour or so will boost them when solar isn't keeping up.
