Most trucks cook with gas, and it would be crazy to idle their main engine. If they need power they use a portable generator.
Having just returned from a week in Portland, I can personally vouch for the amazing diversity and quality of the food truck scene and what it's done for the city and people.
Price? All over the map, from $2 tacos to $6 noodle plates. The best part is that you can literally try a different one each day.
Finally, I know that you are not a racist, but that thing you said about parasites invading was pretty fucking racist. Immigrant entrepreneurs should be encouraged, food trucks are an excellent way to do that, and we get to enjoy amazing food. Portland's restaurant scene is thriving as ever, unless it's Subway you feel sorry for. Heck, many of the best restaurants (Pok Pok) started as a food truck.