I totally agree; just put in what you need.
For the purpose of discussion I'll continue...
Here's one reason someone might want multiple circuits...let's assume 30-amp shore power that we're plugged into and go from there.
So in the morning we decide to crank up the coffee maker and while we're at it we want to run the microwave. [I know, assuming you've even got this stuff but it's just an example
.] We'll assume the coffee maker is 900 watts and the micro is 1200 watts; that's about 8 amps for the coffee machine and 11 amps for the micro and we're well within the 30 amps we have available from shore power. However, we're well above the 15 amps that a single branch circuit carries if the outlets the coffee maker and micro are plugged into are 'daisey-chained' on the same circuit. If we had two circuits (each at 15 amps) we could fully utilize the 30 amps that shore power can supply. Getting 30 amps off of one circuit takes some pretty hefty wire if it's got any distance to travel (look at the size of the typical shorepower cable!).
Another reason for multiple circuits is control; you can shut power down to selected outlets, switches or equipment (like the water heater) and not take down the whole system. And if you only have one circuit and trip the main shore power breaker you lose all power instead of just power on a branch circuit.
I feel more comfortable protecting the wiring in the bus with it's own breaker and each circuit with its own breaker too. That way problems stay isolated to small sections of the system and don't affect the whole shebang. And problems stay smaller in that a problem on a branch circuit trips the branch breaker much more quickly than it would the main breaker on the shore power supply. If fact, the shore power supply breaker could very well be 50 amps (and we've used an adpater to connect our 30 amp shore power cable) and everything in the bus could experience a melt down before the 50 amp breaker tripped (especially since it's doubtful our bus wiring has been designed to handle 50 amps, which a direct short could develop in a heartbeat).
And that brings up my final point, you don't always get 110/115/120 volts at the shore power supply; that depends on how good the wiring and power is to the supply. I've seen voltage below 100-volts in some locations and that means you're going to draw more amperage and subject your system to higher loads. Watts = voltage times amperage in AC or DC systems; your loads always require the same wattage to get their job done (i.e. - a 60-watt light bulb always needs 60-watts regardless of the voltage) so as the voltage drops the amperage has
to go up to keep the equation in balance. On really low voltage big loads like a micro can really draw a lot; in fact we can develop our own mini 'brown-out"!
The cool thing about Skoolie conversions is that we can make them whatever we want them to be. That means different strokes for different folks and no one
thing works for all folks. I'm not implying (at all) that every bus should have an AC system like I've described but if you decide to do a full-on AC system it's nice to have some info available to draw from and that's what I'm attempting to provide here; I hope it helps.