Have you considered the compartment under the bed and slide? It may be large enough if you're willing to give up the storage space. I've seen various motorhomes with spares in storage compartments from factory. With a tape measure you can rule this in or out as an option.
if you don't have tanks between frame rails at rear that's also a convenient place to add a spare. However, with your rear bath that space is probably taken.
You can always go with a hitch mount that works on practically all vehicles equipped with a receiver.
__________________
|