[Richard]

Jerry,

Let's assume that you do not have an echo charger or trickle charger. You should look to see because it makes a big difference in the answer.

If you leave the merge switch on, you will be charging both house and engine batteries. The charger doesn't know what it is connected to, and will try to charge all batteries equally. The issue is that if you loose power, or forget to turn the merge switch off the next time you travel, you will be drawing on both your house and engine batteries. You can kill the engine batteries this way and leave you stranded.

If you leave the merge switch off, you will be charging the house batteries only.