Short version: In fast speech, T plus Y tends to merge to TSH, spelled CH, because of where they are pronounced in the mouth.
Long version: As Adam hinted, this is really a linguistics question. Sumelic linked to a pretty comprehensive answer, but I'll give it another shot.
Here is a broad IPA transcription of one phrase in American English before and after the change:
/wʌtju/ what you → /wʌtʃu/ whatchu
When you compare them, you can see that although they sound quite different to the ear, only one segment changes: /j/ becomes /ʃ/.
That first sound is a palatal glide ("yuh") and the second is a palato-alveolar fricative ("shh"). If you try to make both sounds you'll notice how little your tongue changes position between them.
We notice that in the formal description, two features changed: "palatal" became "palato-alveolar" and "glide" became "fricative".
There are several reasons why features can change, but it's always worth checking if anything in the area could trigger assimilation, the "sharing" of features.
Here, the changing sound is preceded by /t/, an alveolar stop. The "alveolar" is why "palatal" shifted to "palato-alveolar". And a fricative is between a glide and a stop in terms of obstruction of airflow. The stop /t/ "pulled" the glide /j/ halfway, ending up at the fricative /ʃ/. Result: /tʃ/, pronounced "ch".
This assimilation happens easily in quick speech. It's a very natural change and I would not expect it to be bound to one region.
At least, not for phonetic reasons. For sociolinguistic reasons, it might have more stigma or cultural currency in different places among different people. I don't believe, however, there is really any group that avoids it. The answer sumelic linked to includes a clip of the Queen using it.
But writing this change by spelling it with a "ch" is very informal. That spelling is sometimes used as a hint that the speaker is overly casual, not careful in their speech, not educated, or not high-class, even though (again) even the Queen uses it!
Incidentally, despite the uncited sentence that closes the introduction to the Wikipedia article on palatalization, this is a case of affrication. I say that because the sound involved in the change is already palatal before the changes takes place. In fact, it becomes less palatal. But it does go from being a glide to a fricative. The resulting combination of a stop /t/ plus a fricative /ʃ/ is an affricate.
Best Answer
You can stress any word, and it will give a slightly different meaning on what you're saying. The most natural stress for example one is on the word travel, and for number two it's accountant. I'm going to list out the different stresses for the first example, then tell you what they mean:
(For these examples, I'll pretend that them refers to his family)
Someone else wants him to have a different job. Maybe his wife wants him to have a job that earns a lot of money, but he wants a job for the reason above.
He already has a job doing something else, but he wants a different one.
He doesn't want more than one job. Sounds a bit clumsy, if that's what you're trying to say then you'd say "He wants a single job that lets them travel."
He probably already has the means to let his family travel, maybe he has a lot of money saved up, but he wants a job to be his new means of paying for his travel.
Doesn't add any meaning to the sentence, you'd never add stress here unless someone didn't hear you say that the first time.
He doesn't want a job that forces his family to travel.
He cares more about letting his family travel than himself travel
His family doesn't travel yet, he wants a job so that they will be able to.
(Without any context, this is the most neutral place to put a stress on a word)
So in summary, whenever you add stress on to a word imagine what the sentence could be if you swapped the word with something else. That will tell you what information you're really trying to get across to the listener:
"She is my girlfriend" -> You probably thought someone else was my girlfriend
"She is my girlfriend" -> You don't believe she's my girlfriend, but she is
"She is my girlfriend" -> You might have thought she was someone else's girlfriend
"She is my girlfriend" -> Maybe you thought we were brother and sister