Robin Williams is such a great comic virtuoso that it can almost hurt to see him straining to pump life into a conventional, uninspired, sometimes-goofy big-studio comedy such as "RV."

Why does Williams--an improvising wonder whose raging free-association "spritzes" can hot-wire audiences into giddy orgies of mirth--get cajoled into silly stuff like "RV," where he plays Bob Munro, an anxious white-collar guy taking his family on the vacation from hell in a recreational vehicle that often runs amok?

In the movie, he's accompanied by horrified wife Jamie ("Curb Your Enthusiasm's" Cheryl Hines), rebellious daughter Cassie (teen pop star Joanna "JoJo" Levesque) and bewildered son Carl (Josh Hutcherson of "The Polar Express"), and as he drives, rides or even races behind his RV down highways, over mountains and into lakes, teeters with it on a hilltop or gets splattered emptying the toilets, you can't help but sympathize. It looks like a nightmare for both the character and the actor.

Still, Williams does charge up "RV," even if he's a long way from the frenzied genius of "Good Morning, Vietnam " or his mind-blasting stand up routines. Along with director Barry Sonnenfeld and a good cast--especially Jeff Daniels as Travis Gornicke, a cowboy-booted RV expert--Williams gives this manic, preposterous, ill-written romp more laughs and life than it deserves.

At first, it doesn't look easy. The script, by Geoff Rodkey (who wrote "Daddy Day Care" and "The Shaggy Dog") is no help. It manages to be both senseless and predictable at the same time. Rodkey imagines Williams' character, a harassed employee for soft drink company Pure Vibe, forced by his sadistic boss Todd (Will Arnett) to abandon a long-planned Hawaiian vacation with his family in order to help close the merger acquisition of Alpine Soda, a small, early-Ben-and-Jerry-style indie.

But then Rodkey asks us to believe that Bob, determined not to disappoint wife and kids, would decide not to tell them he has emergency work but instead pile them into a brand-new RV and head for Colorado and Alpine Soda, keeping his clan in the dark about his true motives.

Soon, Bob and his brood are rambling through scenic western delights, while the head of the family's nervous road stewardship keeps them in a constant nightmare of backed-up sewage, botched excursions and, thanks to faulty brakes, a frequent runaway RV. But a guardian angel in cowboy boots is also on the road: Daniels' Travis who genially straightens out almost every mess created by the Munros.

Though L.A. Bob initially dismisses Travis as a red-state oaf and also unwisely scoffs at Travis' yodeling country belle wife Mary Jo (Kristin Chenoweth) and their bright home-taught kids; the movie's obvious political point is that blue- and red-staters should bury their cultural differences and pull together. But when "RV" is good, it's because Williams and supporting players like Daniels break right through the nonsense of the premise and the plot. Sometimes they do it with sub-Buster Keaton slapstick and sometimes with what sound like Williams' ad libs, such as his dead-cool basketball thug court rap or his reverse inspirational presentation to the Alpine crew.

Sonnenfeld has directed some good comedies ("The Addams Family," "Men in Black") and some poor ones ("Wild, Wild West" and "Men in Black II"). But Sonnenfeld's prime talent, his flair for high-style cartoonish visuals--the horror house in "Addams" or the ET beasties in the first "MIB"--can backfire when he has weak scripts, inflating the movies and making them seem a bit distorted and daffy.

But Williams keeps "RV" going through sheer comic willpower. As he drives us along, his body seems wrenched by comic angst and his face crinkles into that sweet, pained smile, that gaze of perplexity as the world goes kaflooey around him. Sometimes it works; great comics can triumph over defective vehicles. I just wish Williams could find some kindred spirits and give us more "Good Morning, Vietnams."