I am not entirely clear on what you are saying here. In US household wiring BLACK or RED is always your HOT wire - WHITE is always your NEUTRAL - GREEN or BARE wire is always GROUND. ALWAYS.
Wiring (and everything else electrical or electronic) is color coded so you can know what you are dealing with by looking.
In things like a car you have your RED and BLACK - with RED being (+) and BLACK being (-).
In normal wiring of almost any building or AC electricity WHITE is always NEUTRAL.
In residential wiring you generally see only 4 colors (commercial wiring can have many colors).
BLACK and RED will be hot - this is the incoming power.
WHITE is always NEUTRAL meaning it does not have power until you turn on an appliance - there is not supposed to be any juice on a WHITE wire. This is what is called NEUTRAL or COMMON.
GREEN goes to GROUND - which is for safety with the idea that if something goes wrong the juice will go to GROUND instead of through your body. The shortest distance to GROUND is the route of least resistance - a GROUND wire should have no resistance so power will take that route instead of through you because your body does have resistance.
Each hot circuit is divided up by circuit breakers which protect each circuit should something go wrong. All wires to and from a breaker in a residential situation should be BLACK or RED.
WHITE being NEUTRAL or COMMON means all white wires are connected to each other on a bus bar - so they are all COMMON to each other and not separated from each other by a breaker as a hot circuit would be.
Power comes in through the HOT wires - BLACK or RED - goes through an appliance or light bulb or TV or whatever and goes out through the NEUTRAL or COMMON which takes it to GROUND. This is basically how AC electricity works - different than DC.
So if you connect a BLACK or RED wire to a WHITE wire somewhere/anywhere in a system you now feed juice to the entire system through the COMMON bus or the NEUTRAL leg. This can make the entire system hot with no protection from circuit breakers. This is called REVERSE POLARITY and we see it often in Mexico because many non-electricians do the wiring and don't know not to connect WHITE and BLACK wires.
I do not know how ISB has the inside of their unit color coded as I have not looked inside mine for quite awhile. But I would guess they use the same wiring we do here in the US - and much of the rest of the world. So when you say you connected BLACK wires to NEUTRAL I have concerns that something could be wrong. I cannot see exactly what you did or why. I cannot see inside your ISB unit. But hopefully my attempt at explaining above will clarify what I am trying to explain and you will be able to look at your unit and know if you have done everything correctly.
Everyone traveling by RV in Mexico should have at the very least a basic polarity tester which will tell you if outlets are wired properly with no hot on the COMMON or NEUTRAL. Most electric plugs and outlets in the US today have 3 lugs - blades - whatever you want to call them - a hot - a common - a ground. HOT is always the thinnest blade - COMMON should be a larger blade with a corresponding larger slot in the outlet - GROUND is always round and forms a sort of triangle with the other two. If you only have two blades on a plug then one of them should be larger than the other so you can only plug it in one way. Not every device needs to be plug to correct polarity but most do - hence the difference in the blades to keep you from plugging something in and having the wrong polarity going to the device - say a power drill. This is why we have colored wires and why they are all very specific as to what they are supposed to do within any circuit.
Consider - you have power going to a WHITE wire anywhere in a circuit. Since WHITE is COMMON on the bus bar you now have power going in reverse to every outlet or wired device throughout your house or RV. This is what is called REVERSE POLARITY.
I hope that clears at least some of it up. Even if you do not understand why please understand that the colors are there for a very specific reason and proper wiring is crucial. Also always carry a polarity tester with you and check every and any outlet you might plug into in order to ensure it is wired correctly and you do not put hot circuits on your common in your rig. Polarity testers can be had for less than $10 at just about any hardware store or even maybe Walmart.
Maybe someone else can explain this better than I can. This is why they say to always get an electrician if you do not know. Many people think they know but we continue to see reverse polarity all over the place - even in the US. This can give your RV what is known as a "Hot Skin" meaning your entire rig has power running through it. Touch the wrong thing and the wrong time and you will get zapped.
