When I was purchasing my Roadtrek 190 in 2011 I was looking to add a third battery that would be installed by the dealer after delivery of the Roadtrek to him. When the dealer talked to Roadtrek about this, they were told - at least this is what they told me they were told - that adding that battery would void the warranty from Roadtrek on the electric system. As a result of this, I did not get the third battery. Now, Roadtrek has been installing additional batteries at the factory and has been retrofitting additional batteries along with their solar system (though that has stopped now supposedly due to its distraction from producing new models at the factory). So I am not sure what Roadtrek's reaction would be now to adding batteries at the dealer.
I do know someone who added two additional AGMs by a RT service center to their older Roadtrek and was very happy with the result.
With your extended warranty it sounds like you don't need to worry about RT's warranty.
The whole mod sounds great - all the best with it!