Well… Let’s say you go on the Chinese junk webpage and find a huge inverter for cheap, and you plonk it in your RV and call the electrician to come wire up a bunch of outlets in your RV off that inverter.

Electrician is going to come out, determine the max wattage that the inverter can safely spit out and install a main breaker at that amperage, so that the total load you apply can’t overtax the inverter. Then he might add a small breaker box if the wattage is high enough, and branch some lines out of it, making sure the breaker for each line is low enough that the wires can’t be overtaxed (so they don’t catch fire mostly). That’s all he’s really doing, is installing safety breaks, to protect your wiring.

You pay him, he goes off to the next thing, an you start plugging shit in, and a bunch of stuff starts acting up, because the great deal you got on that inverter was because it’s a square wave inverter, and none of your stuff likes square wave. The electrician hasn’t fucked up anything, you have. The wiring is never going to catch fire because he did his job perfectly, but your shit isn’t working right because you don’t have the right shit…

After a few iterations of stuff like that, electricians learn to simply pass on working with RVs, power walls, backup gen systems, etc. and leave the special stuff to specialist technicians.