I happen to be a crew chief on KC-135s based on the Boeing 707 model. What you are seeing is probably a heat cart (usually really small though), or a pneumatic cart (used as a form of alternate start capability). Could even be a nitrogen cart but since there shouldn't be any maintenance it's probably not.

My bet is that it is AGE equipment... Aersospace Ground Equipment.

Actually probably just a generater cart. Lol for auxiliary power.