What is confusing is that RV's generally follow the national electric code which has colored wire (black or red) as hot and white as neutral (grounded to the chassis). Vehicle convention is that black is ground and red is hot (+12V). I've found often that this confuses owners enough that things can get connected with reverse polarity. The way to be sure is to use a voltmeter before making any connections. 

I have two wires capped and stowed by the batteries labelled "Provision, Electric tongue jack", a red wire and a white wire. I took the battery covers off, the left hand one has a big red wire on the +ve terminal, there is a black jumper from the -ve terminal to the +ve terminal on the right hand battery, and there is a thick white wire on the -ve terminal of the second battery. The two big wires go into a plastic conduit, and the two smaller gauge wires for the tongue jack come out of the same plastic conduit, the conduit disappears under the chassis, presumably to the power distribution box. None of the fuses are labelled for a tongue jack.

Is there a way to tell which of the two big wires is grounded to the chassis somewhere? Motor vehicle convention seems to be -ve to chassis, are trailers the same?