Because you really only need the second battery during cranking, you may be able use a relay to have both batteries connected during starting and running, but have them disconnected while the rig is off. This way they will be there for engine cranking, they will both get charged, but when it is not running you only pull current from one battery.
This may not be a perfect system because the "House" battery will get drained down and then when you go to start the batteries will not be balanced. However, if you keep the demand low, it should be OK. A 3rd battery would be good, but the down side is the extra weight and complexity of adding more wiring. And space.
My suggestion would be to go with the first option and get the best Lithium batteries you can find to fit the application. They will give better performance and they allow you to drain them deeper before things quit working. They are also less than half the weight.
If you are always on the move never staying in one place more than a night, solar is not a great option. However if you are going to stay in one place for several days, solar will keep the battery up during the days while you are not driving.
If you are on the move all the time, your batteries are being used at night, when there is no sun and recharged during the day when you are driving and the engine is charging. Solar would not help.