</font><font color="blue" class="small">( Ya know, I've seen cars burn before and they just don't go up like that one or the ones on TV. )</font>
Having been a fireman, I have seen many cars burn too. From the picture, it looks like the contents of the tank are now on the ground and that is what is burning so vigorously. Any fuel that could have been used to start the fire would have been burned off at this point if it was arson (unless they were stupid enough to put a whole can of gas in the car). What happens in most fires, is the contents of the gas tank expand and blow the cap off, followed by a rush of fuel out the filling nozzel. Depending on model, once all the strapping and rubber grommets burn, lots of parts fall on the floor...the gas tank can be one of them. The attached photo is of a car where the gas tank failed late into the fire.
What is more suspicious is the time of day that it occured. Cars just don't spontaniously combust in the middle of the night. However, the owner could have been warming the car, or just arrived home or....the trailer that WAS parked there could have been involved and left the scene.
Glad your cars were not parked next to it....but you no doubt will have some blistering of your paint. Car fires get pretty darn hot.
NevadaSmith