You'll continue to get "multiple recommendations" to use either the positive cable or the negative cable to install a battery cutoff switch.
In some applications, such as marine applications where there is a potential for the entire boat to become a ground (sitting in the water), or in NHRA cars where the battery location can be "on top of or behind the fuel tank", there are restrictions to use the positive terminal for the BCO.
Electrically, there is no reason, in a travel trailer, that one is "better than or not as good as" the other....
It becomes a matter of which you prefer and/or which is easier to get to.
Keystone installs "most" BCO switches on the positive terminal, but that said, my 2014 Cougar 27RKS came from the factory with the BCO connected to the negative battery cable.....
So.....