So in essence you have 4 batteries in your system and two charging systems.
When running you want to charge all 4 batteries but when not running you only want the solar batteries to charge off the solar system.
What kind of isolator are you using on your truck / rv batteries now?
I am going to assume you are using a Constant use Solenoid to connect the two together with the truck battery connected all the time and the RV battery only connected during running. I am also going to assume your solar controller won't accept a 12volt in from your truck otherwise you wouldn't be asking.
Please correct me if any of those assumptions are wrong.
The easiest way to do this would be to connect your solar batteries to the same connection your RV battery is. While charging they are all sharing the same system anyhow so it shouldn't matter. You will want a 2 way switch that will be able to handle the power running to your solar batteries. This switch would be used similar to how a generator works with three positions. You have connection 1, off, connection 2. This way connection 1 and connection 2 are never connected together regardless of how things fail. You could probably use a Generator switch for this if you can find one that is compatible with DC. Ours at work will switch to the neautral/off position for 10 seconds then go to the other connection but you don't need anything that fancy. We have 480 volts and high amperage running through ours.
Connection 1 would be from the trucks charging system and connection 2 would be from the solar panels. You don't want the solar panels to be hooked to your vehicle power or you could blow up your solar controller. I strongly suggest one of these generator style switches. You COULD use a 2 way relay or better but they can fail with the contacts arced across which would likely not be a good thing.
The other thing to be aware of is what happens to the solar panels when the batteries are disconnected? Some units just shut down but others need the battery to operate properly and can get damaged without them.
You can get fancy and try to use electronics to balance everything out but somehow I don't think you care about that sort of thing.
Note that also in this system when the truck is off your truck battery is powering the truck, your rv battery would be powering your rc and the solar panels would be charging your batteries running the computer. Which I believe is what you want.
You might find it makes more sense to move the RV battery to the solar system and just charge them all either off the truck or off the solar panels. Otherwise your going to lose lights in your rv but still have the computer...
In that case you just use the generator style switch and have it switch automatically between the truck and the rv batteries. Since your rv/solar batteries are the same type and size you should be able to run them together pretty painlessly.