“Diary of a Wimpy Kid: The Long Haul”

1/2 *

(PG, 1:30)

Poor Alicia Silverstone and Tom Everett Scott.

Between “Clueless” and “That Thing You Do!” they separately led two of the most charming and re-watchable all-ages comedies of the ’90s. Cut to 20 some years later and they’ve been relegated to the thankless task of playing the dopey suburban parents in “Diary of a Wimpy Kid: The Long Haul,” a deeply unfunny family road trip film that has gross-out jokes to rival the R-rated “Vacation” remake.

What have they, or any of us, done to deserve this fate?

At least they’re spared having someone vomit in their mouths. This does happen to another character which director David Bowers films in slo-mo to truly give the mud-brown bile its full cinematic due. It’s not 3-D, but it might as well be.

This is technically the fourth “Wimpy Kid” pic, based on cartoonist Jeff Kinney’s popular books about an awkward middle schooler, Greg. “The Long Haul” reboots the series with a new cast (the original kids aged out of the roles), including Silverstone, Scott, Jason Drucker as the “Wimpy Kid” and Charlie Wright as his 16-year-old brother Rodrick, who all hop in the family minivan, with their toddler brother Manny, to travel cross country to attend their Meemaw’s 90th birthday party.

And not only is the Heffley family stuck together for a 47-hour trek to Indiana, but Mom has banned all their devices (even Dad’s), and decided that they’ll rely on How to Speak Spanish CDs, the Spice Girls and a game called I Must Confess to pass the time, with stops at a roach motel and a country fair in between.

Greg has other things on his mind, though — namely how to become internet famous. He already is, but not in the way he’d like. At the start of the film, Greg inadvertently becomes a viral sensation after a restaurant full of people start filming him freaking out that a diaper he inadvertently pulled out of a ball pit is stuck to his hand.

He hopes to relieve himself of this infamy by being in a video with a popular web-based gamer named Mac Digby, who will also be in Indiana at a convention “two inches away” from Meemaw’s on the map.

It’s an appropriately vulgar subplot for a story that seems more disdainful of the messiness of the family experience than celebratory of it. According to this movie, nearly everything about family life is a disgusting, dispiriting horror show from the dreary food to the painful company.

The Heffleys are forced to endure a series of relentless, ever-escalating hijinks, including a recurring thread pitting them against another family where neither come out looking good, and one with a pig just to make things extra crazy. It even ruins a semi-clever “Psycho” shower scene send-up by tacking on some literal toilet humor.

It would be nice to think that Silverstone and Scott might be able to tow any film out of a ditch with their comedic talents. Alas, they’re both stuck in neutral — Silverstone as an underappreciated mom whose clinched smiles wobble as though she’s on the verge of a nervous breakdown, and Scott as the affectless patriarch who is too scared to tell his wife he hasn’t requested vacation time for the trip.

The kids, too, are written as endlessly annoying and barely sympathetic, especially the manic Rodrick, making this already bumpy road trip that much more painful. By the end, you’re almost rooting for the rival family.

The “Diary of a Wimpy Kid” series should go back to the drawing board once more.