The original “National Lampoon’s Vacation” (1983) was a very funny movie–along with “Planes, Trains & Automobiles” (1987) one of John Hughes’s best forays into the comedy of perpetual frustration, smoothly directed by Harold Ramis and played by Chevy Chase so well that it reminds us of what an able farceur he once was. But attempts to copy it have generally proven car wrecks worse than any the hapless Griswold clan experienced in it. It wasn’t long ago that we suffered through “Johnson Family Vacation,” an unacknowledged retread starring Cedric the Entertainer that was brutally unfunny. Now Robin Williams, no less, tries his hand at the formula, with results that are equally dismal. The correct response to “RV” is “PU,” particularly in view of the plethora of jokes about excrement that run through it like a smelly motif (the name the kids choose for the RV is actually “The Big Turd”–and it’s quoted repeatedly!).

Williams plays Bob Munro, a harried executive in a conglomerate owned by despicable Todd Mallory (Will Arnett). He’s forced to cancel a planned family vacation to Hawaii when the boss orders him to accompany him to a merger conference in Colorado. To conceal the reason behind the sudden change in schedule from wife Jamie (Cheryl Hines), daughter Cassie (Joanna “JoJo” Levesque) and son Carl (Josh Hutcherson), Bob rents an ugly RV and announces he’s driving them all to a Colorado lake for a real family outing instead. They all hate the idea, of course, but in spite of the fact that they suffer disaster after disaster along the way, they reconnect as a family during it, until Bob’s original deception is revealed and they have to make up again. They also meet a goofy RV family–the Gornickes, headed by over-friendly dad Travis (Jeff Daniels) and ebullient mom Mary Jo (Kristin Chenoweth)–that they keep bumping into despite their efforts to escape them. It should come as no surprise that the two families eventually bond, too; after all, this is one of those formula movies in which an orgy of wild slapstick and potty humor has to be topped off with lots of heart-to-heart talks, life-changing realizations about what’s really important and a big comeuppance for that nasty boss.

It’s barely possible that a contemporary movie could recapture the spirit of the old 1983 “Vacation,” but this one certainly doesn’t. In terms of the script, things go sour almost immediately with characterizations; Bob’s a befuddled doormat from the get-go, and his kids are pretty much monsters–fifteen-year old Cassie a self-absorbed punkette who thinks her dad’s a jerk, and Carl one of those cocky white tykes who act black and has taken up weight-lifting because he’s self-conscious about being short. (Both are such selfish brats they make you long for a return of corporal punishment.) Even Jamie seems an unsupportive sort. But one might be willing to put up with this largely unsympathetic bunch if the “adventures” they went through were amusing. Within ten minutes, though, we’re treated to a cruelly overextended bit about Bob trashing the street by trying to turn around the RV (flatly directed, as the whole picture is, by Barry Sonnenfeld), and within twenty there’s a literally execrable, and hideously long, sequence involving Bob’s attempt to siphon off the sewage from the vehicle, which ends in a geyser of fecal matter. By this time you may already be tempted to flee the auditorium, but rest assured that although the trip takes the family up into the Rockies, things still go pretty much downhill from there. Indeed, the appearance of the Gornickes–though they’re initially portrayed as white-trash cartoons–actually improves matters; they may be nothing more that imitations of Randy Quaid’s down-home clowns from “Vacation,” but they’re more fun than the Munros, whose descent into familial bliss at the end trumps all the clumsy slapstick that’s preceded it in groan-inducing crumminess.

Through it all Williams does his familiar shtick, which over the years has become increasingly annoying (it’s now merely a question of whether he’s worse when he does sappy comedy or sappy drama), while the rest of the Munros–Hines, Levesque and Hutcherson–remain pretty much insufferable even when they turn “nice.” Daniels and Chenoweth actually do better, even if they’re encouraged to mug it up rather fiercely, and their kids–Hunter Parrish, Chloe Sonnenfeld and Alex Ferris–are fairly pleasant company, too. (Maybe the movie should have been about them, though it might just be the smaller dose that keeps them tolerable.) On the technical side, too, the picture is sub-par: the many slapstick episodes involving the RV are especially clumsy, and the whole thing has a vaguely tattered look that doesn’t speak well for Michael Bolton’s production design or Fred Murphy’s often overbright cinematography.

There’s a sequence in “RV” when the Munros are stranded in the desert beside their vehicle, and for a brief moment your memory might flash back to the recent remake of “The Hills Have Eyes” and you may wish that the mutant family from that movie would suddenly descend upon them and end our misery. But that, of course, would be too cruel–especially on the mutants.