To best describe "RV" would be to compare it to the "Vacation" franchise of comedies. No, not the classic 1983 original, but more the defective 1985 installment, "European Vacation." You know, the one that's kind of funny, has its moments, but doesn't quite lace together properly? That's "RV."

"RV" marks a return to the big screen after a four year absence for director Barry Sonnenfeld. I can't say the film plays to the filmmaker's favorite type of high-spirited, silly cinema ("The Addams Family," "Men in Black"), but it isn't exactly "Men in Black II," and we can all be thankful for that. "RV" is the skim milk version of a Sonnenfeld experience, with the filmmaker covering every joke very safely, so not to offend. Frankly, I love when this director offends, and there are some great moments in "RV" that lean toward family wisecrack nastiness and cruel realities about marriage. There's a wealth of ripe material just waiting for the production to take advantage of, from Robin Williams's rare willingness to goof around to the great expanse of the whole road trip plot.

Where Sonnenfeld and writer Geoff Rodkey ("Daddy Day Care") would rather go is into the toilet. There are various riffs on bathroom gags, most prominently a sequence where Bob wages a losing battle with the vehicle's human waste disposal tubing, ending with the gunk literally raining down on him. Pandering to the gross-out mentality is a shame, but Sonnenfeld can't help himself. He knows it works for the kids, but it murders the lovely spark established in the early going. I would rather watch Cheryl Hines sing along hilariously to "G.T.O." than hear Robin Williams crouching behind a tree hoping to convince his family he has diarrhea. Criminally, Sonnenfeld feels the opposite.

Regardless of the bathroom fixation, it is fantastic to see Williams playful again. Now fully comfortable in the sad sack dad roles he prefers, Willams lets loose some of his old energy under Sonnenfeld's watch, getting ample time for delightful physical comedy and some under-the-breath zingers. He has a great partner in Hines, who sparkles in the traditionally nothing mom role. More help is provided by Jeff Daniels and Kristin Chenoweth, who play a folk singing, rival RV family looking to bond with the Munros, turning up at all the right moments, much to Bob's horror. The cast is exceptional, and they struggle to keep Sonnenfeld's directing apathy at bay.

Much like "Vacation," the underlying theme of the picture is the preciousness of family time, and how work schedules can rob parents of it. Where Clark Griswold grabbed a BB gun and took his family time by force, "RV" becomes all mawkish and gooey – not the best method for belly laughs. If Sonnenfeld was making a big, brassy comedy, he's failed by desperately trying to pluck heartstrings. And if he's after a schmaltzy family comedy, he's failed because the jokes are weakened by the sentimentality. "RV" has some ideas right, and there are some enthusiastic laughs; however, it doesn't come together crisply enough in the end to stay memorable.