Just thought I'd share a few of my favourite food trucks in London that are owned by Americans that make some great food.
Cheese steaks:
Liberty Cheese Steaks
The owner is from Philly, and they make some great cheese steaks. They couldn't get Cheese Wiz over here so they make their own homemade Wiz, and it's great! They also do a Philly roast pork sandwich. You can find their truck parked at Spitafields Market, and other locations.
BBQ:
Prairie Fire BBQ.
The owner is from Kansas City and they make some awesome BBQ, Kansas City/St Louis style. I'm not sure where they are regularly, but I bet you could check their website. They make one my most favourite BBQ sauce I've ever had. They sell it by the bottle. You can also find the bottles of sauce in some meat markets around London.
Burgers:
Blecker Street Burgers.
Ok, there are actually many good burger joints in London, but I mention this one, as the owner is from New York. Some of the best burgers I have ever had. They have a stall in Spitafields market and a semi permanent stand at Canary Wharf. They may have other locations and a truck too, not sure where it is, but the Internet will tell you.