90% of the time i have the rear doors closed leaving just the side door open which is where the TM is situated. During the hot summer months, if on a customers drive will leave the rear doors open but if i cant get on customers drive or am on commercial job i will direct an air mover or paddle fan towards the TM just to aid air movement just in case. Only once have i ever had a problem with a TM engine overheating, this was a Kawaski water cooled engine, I was outside a night club in Leeds parked on a slight slope, the side door had 3/4 closed so the air in the back of the van just got steadily warmer (was a blistering hot day aswell). The TM stoped, i went outside and found the engine had cooked. Cost about £500 to repair the engine.
I know a company that had a Prochem Performer 405 mounted in a van that had 4 rotating vents on the roof. They drilled through the floor of the van and had the vac / solution / water inlet connected to vac ports, pressure outlets & a water inlet mounted under the rear of the van along with TM exhaust outlet. This was not done for security purposes though. It was for the purpose of sound. They were getting residents complaining of noise when cleaning commercial premises at night. They also had the rear of the van sound proofed in some way.
Richie.