Maybe you should consider a diesel generator since gas is not available there if you want to power such a large load. One of the drawbacks though is fuel storage
Powering 400 amps off of an LP generator is a large load and will burn through a tank of propane pretty quickly. Maybe you could decide which circuits you absolutely have to power (such as fridge, lights, heat/AC, etc) so you can go with a smaller generator and your propane supply will last longer.
I personally went the portable generator/manual transfer switch route since I don't have a gas line coming in, nor do I have propane (other than enough to power a gas grill). Everything in this house is electric (including heat, which is expensive). The circuits I am powering with the generator is the fridge, downstairs heat, interior lights (including the circuit that powers medical equipment most importantly), an most of the wall sockets (TVs, computers, and window A/C units). Everything else, water heater, stove, microwave, washer/dryer, upstairs heat, and garbage disposal will be non-functional during a power outage.
Fortunately we have city water (no well pump) and we do not have a leaky basement (no sump pump required).
I bought the generator during the October snowstorm and waited a good two hours in line. Lowes had a shipment of 800 generators that day and they had 46 left by the time it was my turn. I bought it sight unseen. Of course the lights came on a couple of hours later after being out three days (did not have to use generator). Had I not bought the generator, I'd would have been out a good two weeks.