I see where, Joel Kamentz managed to write:

There are plenty of ways for current to leak to ground if only the
negative terminal is disconnected.  Between the battery post and the
bottom of the battery tray, humidity, grunge, corrosion can allow a
medium resistance to ground.

It's not the knife switch itself, it's the quality of it.  A knife
type switch will work to disconnect either or both lines, (both is
best).

Frank's answer is good, but leaves out an important point.  In 12V
systems, the negative terminal is connected to ground, so a
leakage-type connection to ground (through the battery case, rust,
grunge, etc. that Frank mentioned) means a completed (albeit poor)
circuit.

That would add a small measure of safety to the disconnect, but also
means one more thing to go wrong, especially if the knife switch is
cheap.

One other thing about knife switches - they are designed for stationary
installation, and sometimes work well under those circumstances.  In
situations where they can bounce around (i.e. a pop-up on the road) or
have metallic things dropped on them (tools, metallic coat hangers,
spoons, etc), they are just a problem waiting to happen.

Austin

I work with UPS battery systems quite a lot in my work.  Regarding
switching batteries on a RV, it really does not matter whether you
switch the positive or negative.  In higher voltage units, i.e. over 50
Vdc, then rules change you switch the negative.  The reason for
switching the negative rule applies as over 50 Vdc there is a potetial
for peresonal shock so you first remove the reference to ground.

I like the advise one mentioned of disconnecting battery power when not
in use via a battery switch and not the sensor alone.  Switching the
sensor alone leaves a risk of forgetting and this could be a liability
if you forget to recconnect and had a gas leak!

Regarding the battery draining down over time some have mentioned, the
biggest culprit we often find is battery leakage between terminals
outside the casing through dirt and grime.  Not too familiar with
internal controls in RV's, but I imagine the converters, refrigerator
and possibly heater controls may have some parasitic loads.   Also a
battery loses potential just sitting around, as much as 1% per day
depending on the state of charge and external resistance.

My suggestion is to keep the batteries clean.  Use a small trickle
charger designed to maintain 12Vdc when not in use.  The alternative is
to place the battery on a charger once every three months to keep them
from sulfating.

Seasons greetings and happy camping

JerryB