Deliberate mistake: At the end, there is no way that the RV managed to roll over and flatten the vehicles in its path.
Continuity mistake: When they arrive at a campsite to take in the view of the lake that Bob saw with his family as a kid they leave the door to the RV open with the stairs down. After some dialogue the RV rolls into the lake with the stairs up and the door closed.
Character mistake: When Bob Munro (Robin Williams) is behind the tractor, the speedometer shows that it is below 10mph. He then states that they are going twelve. How could he if they didn't reach 10mph yet?
Visible crew/equipment: When the Gornickes and the Munros are eating outside at the RV park, you can see a crewmember reflected on the Gornicke bus.
Continuity mistake: When Jamie and Bob are sleeping in their bed, the things from the bedside table have been changed.
Answer: In the world of "make believe", they used "movie magic" to zap the RV out of the water and on to dry land - with no mechanical issues resulting from being submerged. In the real world, someone called a tow truck - perhaps AAA - and the RV was pulled out of the water and it suffered water damage and needed some repairs. This movie was presented as being "real life." Bob left on a bicycle to "try to find help." Near the end of the movie, Carl said that the RV "spent two days under water and they had to fish it out." He didn't say who "they" were. A fishing pole would not be strong enough to reel in a large RV, so I think it is safe to conclude that a tow truck was used to pull the RV out of the lake.
KeyZOid
It should be noted that "fish it out" is a common phrase to mean pull or take out, especially after searching. When people use the term, they're never taking about using a fishing pole. But often when people post questions like this, they're asking for an in-film explanation in case they missed (or didn't understand) something. If no in-film explanation was given, a reasonable speculation can be given. You don't need to remind people the movie is a movie. If the in/film explanation is uncharacteristic to real life, then one can point out that in real life it wouldn't happen that way.
Bishop73
It was meant to be ironic.
KeyZOid
There was no irony, but this isn't the forum for irony anyways.
Bishop73
I guess I failed miserably... but wasn't the original question rhetorical?
KeyZOid