Think “National Lampoon’s Vacation” retooled for the “Hangover” generation and you’ll have a good idea of what “We’re the Millers” is like. Like the Chevy Chase perennial, it’s a catastrophe-filled family road movie, but in this case the family is fake, the trip is a drug-smuggling journey and the script is crammed with the sort of gross-out gags, sexual humor and rough language that would have made John Hughes queasy. Though incessantly raunchy and crude, the movie is intermittently funny. Too often, though, it just spins its wheels or drives into a ditch, figuratively speaking.

The sparkplug of the venture is David Clark (Jason Sudeikis), a low-level marijuana dealer with an endless stream of snarky one-liners. When thugs steal the cash he owes to his supplier Brad Gurdlinger (Ed Helms), his only chance to save his skin is to agree to transport a shipment of weed across the Mexican border. The scheme he cooks up to avoid detection is to play a nerdy dad driving his vacationing brood to Mexico in an RV and concealing the cargo in it for the return trip. And so he persuades Rose O’Reilly (Jennifer Anison), a cash-strapped stripper who lives down the hall of his ratty apartment building, to play his wife. For kids he adds dweeb-next-door Kenny (Will Poulter) and hard-as-nails runaway Casey (Emma Roberts). Looking as whitebread and unthreatening as can be, they’ll easily pass by the border crossing without so much as a glance from the guards.

Problems begin almost immediately after they acquire the huge shipment from a one-eyed giant (Matthew Willig), when they’re accosted by a portly cop (Luis Guzman) demanding a bribe payable either in cash or sexual favors. And at the border itself they arouse suspicion anyway, after a conversation with doofus couple Don and Edie Fitzgerald (Nick Offerman and Kathryn Hahn), who are also in an RV. But they get back into the US, only to have their vehicle break down, bringing Don, Edie and their daughter Melissa (Molly Quinn) to the rescue. That introduces a long digression with lots of crass sexual overtones involving David, Rose, Don and Edie as a clumsily “swinging” foursome and Kenny and Melissa in a puppy-dog romance. Like them, a bit about Rose’s supposed baby is ill-conceived and ends with a thud.

That’s just the beginning. The family is soon being pursued by One-Eye and his cartel boss (Tomer Sisley), which at one point leads Rose to do a strip routine for the villains—a sequence that’s more unseemly than funny. But it’s certainly topped by a grotesquely protracted—and extraordinarily graphic—sequence in which Kenny is bitten by a tarantula in a most inconvenient anatomical location. There’s also a revelation about Don that plays into a big finale.

Of course, the smuggling plot isn’t enough. The script also has to play what has become the almost obligatory card in movies nowadays—the “what makes a family” theme. As David, Rose, Kenny and Casey continue their odyssey, they bond, growing protective of one another over the course of their misadventures. By the close they’ve inevitably transformed into a real family, linked not by blood but by love. As if anyone couldn’t see that coming!

To be fair, “We’re the Millers” does give Sudeikis a chance to showcase a talent for playing a shifty but lovable cad who tosses off put-downs with the quickness of a stand-up comedian. And Anison is as always an agreeable presence, even though the striptease stuff has more than a whiff of exploitation to it. Roberts is fine without doing anything special, but Offerman can’t make much of his dull role, and Hahn balances his underplaying by going to extremes; Helms, meanwhile, is surprisingly unfunny—though predictably broad—as Clark’s drug impresario. The one true shining light of the movie is Poulter, whose performance as big-hearted, naïve Kenny is its most enjoyable element. It’s a pity he has to go through that spider business.

Technically proficient without being outstanding, “We’re the Millers” represents the current state of Hollywood comedy to perfection. It aims for the funnybone but more often hits the crotch. If that’s to your taste, you may find it hilarious. If not, it’s a trip you won’t want to take.