Living in a trailer park because...
...they spend all their damn money at Gilley's!
Bud makes pretty good money at the oil refinery, by his own words somewhere around $18/hour. Sissy also has a job with her parents at what looks like a rag-tag garage of sorts, with towing service (which we see her performing).
Anyway, you have a 2-income household, and Bud goes and buys what is probably a $10,000 single wide trailer (and the best part is you can move it if ya want!). Sissy goes bonkers upon seeing the trailer, like she just won the lottery. I mean, that's fine, they are happy at the moment, but I'm here to tell you people, this is such a familiar pattern of behavior for many many people. And, it's a country-wide phenomenon, not something limited to Texas in any way.
So many people spend almost their entire income at bars, meet a partner there and 'live like pigs' ever afterward, and are eternally miserable as a result, just like this movie depicts. It's even well known at the time this movie was made, that many bars would actually be willing to cash your paycheck for you, so you didn't even have to waste time going to the bank!
But more on the irresponsible way these people spend their income. It's not uncommon where I live to drive by any run down trailer park and see a beautiful pick-up truck that obviously cost twice as much as the home they live in. Maybe sometimes you also see a boat w/trailer, or jet ski's etc...just bizarre to see people willing to live like complete shit so they can have a few toys and plenty of booze.