I see no real reason the knife switch has to be on the negative side, other than a possible safety issue. If the knife switch was on the positive side, it might make contact with a part of the metal chassis that is connected to the negative side when switched off.
But since electrons flow from negative to positive, it is more proper I suppose to disconnect the negative side.
But there are millions of DC sources switched at the positive side.
If it were me, I'd use a marine-grade disconnect switch rather than a knife switch.
Typically the marine ones connect to the positive side. If you have batteries in parallel, ideally you want to disconnect the batteries from the coach and each other.
If you leave the batteries in parallel, they will equalize the state of charge between them, so if one battery goes bad, it will drain the other.
There are marine versions of a battery switch that would be ideal in this situation, and the switch has positions for OFF (both batteries disconnected), BOTH (both batteries connected), 1 (only one battery connected), and 2 (the other battery connected).
Its not really something I see much in RVs, but they are very common in dual battery boat setups. The only issue is the battery must be connected to keep it charged, so if the switch is in the OFF, 1, or 2 position, one or both batteries will not be connected to the charger.
But say you want to charge your batteries once a month in the winter, simply put the switch to BOTH and plug your coach into AC so the charger becomes operational. Once you are finished charging, unplug AC and put the battery switch back to OFF.
The 1, and 2 positions would probably not be used much - unless you have discovered a bad battery during a camping trip and want to isolate it. The primary reason for the 1 and 2 positions are for boats so that they can keep a fully charged battery disconnected in reserve. That way, if they drain the battery down, they can flip over to the other battery and start the engine.
However, since RV coaches don't normally start engines with the house batteries, these positions might not be used much.
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