What does the word trucking mean in the sentence "This chapter introduces you to your digestive system and explains exactly how your body digests the many different kinds of foods you eat, all the while extracting the nutrients you need to keep on trucking’. This is from a book called nutrition for dummies.
'Truck' means to 'proceed, go, move, stroll' - by truck or otherwise (OED, U.S. slang)
1925 C. R. Cooper Lions 'n' Tigers v. 109 One by one the big wagons were trucking toward the first smoking torch at a corner of the grounds.
1941 J. Steinbeck & E. F. Ricketts Sea of Cortez xxiv. 237 We said good-by to Tiburoń and trucked on down toward Guaymas.
1979 United States 1980/81 (Penguin Travel Guides) 148 You'll still find plenty of people trucking through the streets in flannel shirts, blue jeans, cowboy hats, and boots.
Keep on Truckin' is an American idiom popular in the 1940's Swing Era. The phrase has nothing whatever to do with exhortations from the International Brotherhood of Teamsters. It was used by 'Hep Cats' who dig (understand) jazz and swing music (Tommy Dorsey, Duke Ellington, Glenn Miller, etc.). 'Truckin' has a definite movement. It is a hip-tossing, index finger-waving style of walk performed by hep cats who are 'in the groove' (in tune with the music). The phrase 'keep on truckin' (not 'trucking') still lives even after the demise of the Big Bands and Swing music, but the hip-tossing, finger-waving movement does not. The meaning today is to persist in pursuit of one's purpose (Merriam-Webster.com), just as the phrase meant in 1940 to persist in the pursuit of Swing music and Swing Dancing.
Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) treats the 1970s-era phrase "keep on trucking" as quite distinct from the jazz-age slang term truck:
keep on trucking v phr by 1972 To carry on; continue what one is doing, esp working, plugging away, etc. =KEEP ON KEEPING ON [which is listed as "by 1970s," with the meaning "To persist; hold one's course"]
truck 1 v by 1681 To carry; haul; lug: Why are you trucking all that weight around? 2 v jazz musicians by 1925 To leave; go along 3 v 1930s jive talk To dance the jitterbug; esp, to do a jitterbug dance called "Truckin"
Chapman & Kipfer's view notwithstanding, there is at least one crossover occurrence of "keep on trucking," from the Holy Modal Rounders' 1964 version of "Hey, Hey, Baby":
You can do what you, you can say what you
But just keep on trucking my blues away
The face-value meaning here is evidently "continue to transport my blues away from me," but there may be an undercurrent of sexual innuendo at work as well, especially since the song is about using cocaine and about "what every woman wants from every man."
Robert Crumb used the phrase "keep on truckin'" (taking it from Blind Boy Fuller's depression-era song "Truckin' My Blues Away" , according to Wikipedia) in a page of Zap Comix in 1968, and it became a catch-phrase soon thereafter. Crumb continued to use the expression multiple times in subsequent comics that he drew.
My impression is that "keep on trucking [or truckin']" is not especially closely connected to the jazz-age or jive-talk meanings of "trucking" that Chapman & Kipfer identify, but rather to the rather transparently euphemistic rhyme-slang term "trucking" that Blind Boy Fuller used in his hokum blues song from 1937 and that the Holy Modal Rounders repeated in their song in 1964.
As used in everyday U.S. speech today, however, the expression "keep on trucking" has lost any sexual innuendo it may once have possessed.