The most popular batteries vent corrosive gasses. Take a look at the stock start battery location in your bus. Do you notice anything unusual compared to other areas of the bus? All that rust & corrosion is a result of the batteries working & gassing.
How well do electrical connections work when they are corroded?
Now, do you want that expensive electronic device (that is chock full of sensitive electrical connections) in that environment?
Oh sure, you can get the pricy AGM's to minimize the corrosive gases. You can add forced ventilation too. But why not avoid the problem rather than chase symptoms?
If you properly locate the inverter away from the battery environment, you will lessen the ventilation requirement for the battery compartment (could possibly go with convection rather than fan forced).
Personally, I'd opt for the Trojan deep cycle lead-acid batteries & use a fraction of the money saved to enclose/ ventilate the inverter & batteries seperately.
But that is just my opinion
Oh, about the converter, I'd pass on that & invest in a really good inverter & battery bank to eliminate the need for the converter.