I bought the switch from Kennects Auto Parts for Less than $15. The Key is removable. You can also find them on Amazon. You will also need a short negative battery cable.
Check to make sure you have room inside your battery box and drill hole in lid. Mount to NEGATIVE SIDE OF BATTERY. One side of switch to battery and the other to the trailer. I used silicone when mounting the switch to the battery box so water would not get in.
I have so many little lights that stay on in the trailer my two deep cycle 12 volts would drop to 12.3 volts in about two weeks without being plugged into shore power. Now, the trailer has not been plugged in for almost two months and batteries are still at 12.76 volts which is fully charged.