Few comedy franchises in film history have a more bizarre saga than the “National Lampoon’s Vacation” series.

Yup, the original Harold Ramis/John Hughes film from 1983 was a low-rent comedy classic, with Chevy Chase’s bumbling Clark Griswold hell-bent on taking his family from Chicago to the California theme park Wally World.

After that, things got really weird. Different actors of various (and inconsistent) ages played the Griswolds’ children, and the quality of the films ranged from dreadful (“European Vacation” in 1985) to sporadically funny (1989’s “Christmas Vacation”) to unwatchable (1997’s “Vegas Vacation”).

Now we’re subjected to “Vacation,” with Ed Helms as Clark’s grown-up son, Rusty, and it’s a vile, odious disaster populated with unlikable, dopey characters bumbling through mean-spirited set pieces that rely heavily on slapstick fight scenes, scatological sight gags and serial vomiting.

Pack the car and take the whole family!

The usually likable Helms does a variation on the cheerfully clueless, lovable semiloser persona he’s often played on TV and in the movies.

Rusty is a pilot for a bargain-basement airline. He’s married to the lovely Debbie (Christina Applegate), who seems a bit restless in their relationship. Skyler Gisondo is uber-sensitive older brother James, who keeps dream journals, plays the acoustic guitar and always seems to be on the verge of tears. Steele Stebbins is little brother Kevin, who hates his older brother and is constantly attacking him physically, berating him verbally and wishing death upon him.

The little monster tormenting his bigger brother shtick is funny for about 30 seconds. After that, the “joke” gets really old and kinda sick.

With the Chicago area-based family stuck in a vacation rut of staying at the same cottage in Michigan year after year after year, Rusty has a brainstorm: They’ll drive to Wally World, just like the Griswolds did back in the day. (There’s a mildly funny bit in which Rusty and his kids talk about the original vacation, and James says he’s never even heard of the original vacation, and Rusty says this vacation will stand on its own.)

Let the wacky road-trip antics ensue! There’s a callback to the famous scene from the 1983 film where Clark leers at a beautiful blonde (Christie Brinkley) who zips by in a red convertible – but this time around, the payoff is brutally unfunny. A stopover at Debbie’s college results in some really bad acting from actresses playing sorority girls, some cringe-inducing reveals about Debbie’s past and a drinking-and-vomiting scene because we haven’t had enough of those in crass comedies.

Leslie Mann cameos as the grown-up Audrey, who’s married to a chiseled idiot (a game Chris Hemsworth) and is profoundly unhappy. Chevy Chase and Beverly D’Angelo show up late in the story to reprise their roles as Clark and Ellen Griswold – but writer-directors John Francis Daley and Jonathan Goldstein have turned them into sad, unfunny old people. (Clark in particular seems to be utterly and completely insane. He was always a goof, but geez this is his fate?)

“Vacation” earns its R rating, thanks mostly to language. But whether it’s Rusty learning the details of Debbie’s promiscuous past, or Rusty and his older son having one of the more uncomfortable conversations about sex you’ll ever see on the big screen, the frankness far outweighs the actual laughs.

I’m all for bawdy, politically incorrect, wildly inappropriate humor – when there are consistent and genuine laughs to be mined from the material. This just sits there like a steaming pile of stuff you walk around. Rated R.