The ER should already have plenty of outside combustion air available, not sure why it's not recommended to locate there.
My biggest recommendation on location would be to keep the exhaust
hose as short as possible. That thing gets dangerously hot. I don't mean hot like the outside of your engine block, I mean closer to cherry-red hot. It shouldn't pass through or anywhere near any combustible material.
You can always figure out a way to get combustion air in, and duct the heater intake and output from/to where you want it.