You see a lot of semi-home made dive team and scba mobile fill stations for sale. I think the problem is they are really fun to plan, fairly fun to build, but of a lot less use than it they seem from afar.
First thing to do, when considering building one, is ask yourself what the maximum number of tanks you might ever need filled on the spot, then stop and think whether it might be cheaper and easier just to buy that many tanks. As you have noticed, it takes a lot of power to run a compressor big enough to fill tanks quickly. Most portable compressors will fill only 2-4 tanks an hour, hardly enough to bother with.
Another is that a fire or emergency site is not really a good place to be filling! There can be smoke or other pollution in the air (including exhaust if the compressor is IC powered, as it will probably have to be to fill tanks quickly enough to be useful) and the compressor operator is exposed to all sorts of distractions which can be very detrimental to the safe filling of tanks.
One possiblity to consider is to forget the compressor, which is often more trouble than it is worth, and go with just a mobile cascade. Even better, add a booster, which lets you make full use of the air in the cascade, and often, provides faster fill rates too.
