I do not think there is a specific name for those melodies (I know them well, and hated them as they used to wake me up twice a week when I lived in Kanagawa-ken :) ).
They also seem not to be a standard, but to depend on the area and the specific town/neighborhood.
This Wikipedia article simply refers to them as {} ( "electronic melody"). In particular, quote:
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This is another article that explains how these melodies really seem to depend on the area and town (as well as the fact that the trucks make a sound at all). Also in this article, I don't seem to find any specific word to describe this sound. It is broadly referred to as {}.
This article also report some commonly used melodies such as: ,,, and (some of these are also referred in the Wikipedia article above).