Firstly is it a 24 V system or 12V? If it's 24V then the two 12V batteries have to be connected in series to give 24 V. Connecting in parallel will give 12 V.
That means positive lead on truck goes on positive of battery No.1. The short link/lead goes from negative terminal of Battery No.1 to the Positive terminal of battery No.2. Negative lead from truck chassis side-member goes onto negative terminal of battery No.2.
The positive and negative terminals are different sizes and the clamps on the leads to go on them are also differnet, so it should not be possible to mix up + and -.
JD
June 2010