JUST when you thought homeless ERIN MORAN couldn’t sink any lower, the “Happy Days” star got into an ugly street brawl with her mother-in-law after trying to sneak into the old lady’s home!

Now those close to Erin fear that her out-of-control lifestyle is going to kill her.

“Erin’s going down for the count,” a close family friend told The ENQUIRER. “If she keeps this craziness up, she’s going to end up dead.”

As The ENQUIRER recently report­ed, 52-year-old Erin – who played cute Joanie Cunningham on the popular ’70s and ’80s sitcom – and her hus­band Steve Fleischmann, 45, had been living in an Indiana trailer park with Steve’s 72-year-old mom. But the couple got the boot thanks to Erin’s nonstop party­ing.

After weeks of bouncing from one hotel to another – and blowing through nearly all of their money – Erin and Steve asked his mom if they could move back in with her. She refused.

But rather than honor her wishes, the source continued, Erin and Steve hatched a plan for him to move in and for her to sneak in after his mom went to bed. But the ruse fell apart when the elderly woman caught Erin trying to slip into the trailer – and she exploded!

The situation got so tense that Steve’s mom threatened to call the police if Erin didn’t leave imme­diately, said the source.

“She finally did leave, but Steve’s mom was right on her heels, chasing her out of the trailer, shouting at Erin as she made her way across the yard to their car.

“Steve’s mom was standing in the street in a nightgown and hair curlers, and the two women were screaming profani­ties at each other.”

When it looked like the situation might escalate to physical violence, Steve finally in­tervened and managed to get Erin into their car.

“It’s evident that Erin’s living situation has become more dire than ever before,” added the source. “She’s desperate and quickly run­ning out of options.”