"simple truck" is a temporary name, because I still could not find a translation of the french "porteur" category, i.e. trucks like this:
http://www.obrist.ch/images/Camion-Gay.jpg
http://www.senacyt.gob.pa/g_metrologia/cenamep/artes/camion.png
http://www.marbriella.it/comitato/foto/camion.jpg
...

The garbage trucks can go in that category, though we could also create a special group for them.

Tankers and flatbed go either in this category or in "trailer truck", depending if they have a trailer or not.
A trailer truck with a tanker or flatbed is usually the same truck as another trailer truck, so for these there is no need of a special category. And so I find strange to make lots of category for the non-trailer trucks.