Bank Shot — Donald E. Westlake

"What the hell is it?" Dortmunder said.
"It's a trailer," Kelp said. "What they call a mobile home. Didn't you ever see that kind of thing before?"
"But what the hell is it?"
"It's the bank," Kelp said.

The bank has a tough safe, seven guards, and a police station seven blocks away. An ordinary gang of robbers would turn up their noses at this sort of heist, even if the bank is temporarily housed in a trailer in an otherwise vacant lot.

But John A. Dortmunder has been blessed (or cursed) with imagination. With the aid of Kelp's nephew (a youthful ex-G-man who collects old detective pulps and dreams of a life of crime), Dortmunder and his inept cohorts plan to heist a truck, hook up the trailer, and make off with the bank, the whole bank, and (with luck) nothing but the bank.
But nothing is ever that simple...