Re:Generator and Propane Mounting Problem
Can't predict what codes would apply, but we have installed tanks (NGV high pressure type) adjacent in a utility compartment that contains water heater, generator and such, with a solid separator wall between the equipment and tanks, the tank area is then heavily ventilated with louver type panels, with the lowest tank above the lowest louvre such that any gas build up is more difficult to occur or settle in the lower area.
The more open the area of the tanks is the better as stated above. If its in an enclosed area, you can use marine type gas detector as an alarm for a shut off system. If you google marine applications for this type of design you'll find a lot of info.
I'm not sure of codes in the states, but rear bumper mounted tanks of any type would not be a choice for us. Is simply too easy to have a rear end collision these days and I'd never want all that fuel, whatever type let loose from a split tank. Its one thing to have a small leak and have good ventilation to disperse it, but a big sudden rupture would be an instant conflagration IMHO.