First question I would have is the two deep cycle batteries, are they 6V? Or two 12V? If 6V then the negative on one should go to chassis ground, it's positive should go to the negative of the 2nd 6V, and it's positive should go to the booster solenoid on the wall. There should be another heavy cable going to the genset from either the positive on the solenoid or from the positive of that battery set.
If they are 12V batteries, the wiring would be the same, only they would be wired positive to positive and negative to negative as well as having the chassis ground and solenoid cables.
Sometimes, the RV will have the starting cable from the genset connected to the starting (chassis) battery instead of the house batteries.
The negative going to that solenoid must be wrong. There should be positive 12V there.