You are right – the AGMs and the deep cycle batteries can’t be hooked up to the same charger. To charge the deep cycles in the wheel well if they are hooked up to the truck alternator for charging, means you would have to run the truck engine to charge them. Sort of inefficient if you happen to be parked somewhere for days and not needing to drive.
And I would never put that type of battery in a place where I could not inspect them – maintenance free or not. If they are ahead of your wheel well in your truck bed you would have to lift the camper to check on them.
I would do as Mike suggests, sell the deep cycle batteries, and go with all AGMs. As for locating the third (or fourth) AGM, the wheel well is a possibility for an AGM.
I would suggest that if you really are intent on getting a third or fourth battery, hook all your batteries in a single 12 volt parallel system, and buy identical batteries all at the same time so the batteries in the camper and the extra(s) all have the exact same charging characteristics. Not a good idea to mix old and new batteries if you were to buy the extras later. And put them all on the same charging systems – your solar charger (set for AGMs) and your Onan/120v converter. That is a lot simpler system to put together and maintain than mixing battery types and involving the truck alternator.
Look at Lifeline AGMs – they have served me well.