while I suppose you could provide a different power source to a specific breaker/circuit, such as for the Air Conditioner, thereby 'bypassing' the Main Panel's usage of it's own 30amp Main power source, it can get problematic.
If you always have access to this 'extra' 15/20amp power supply, and it's not effected by the RV's own 30amp power source, then you could certainly provide the power directly to the specific device, but it would have to be DIRECTLY, since there's no way to provide two different power sources at the same time within a Breaker Panel(to the power buss that the breaker itself attaches to), since multiple breaker all attach to the same power buss.
You could think about adding a transfer switch, though, which solves the problem, allowing you to use the 'additional' 15/20amp power source, when available, yet having the continued power to the circuit when you only have 30amp power.
I used a simple relay/transfer switch for my own needs last year, when a problem with my Inverter caused my sub-panel of 15amp Inverter breaker/circuits to only have power when I was on Shore or Generator - the Inverter was not working, nor it's internal 'pass thru' transfer switch.
This simple SPDT relay(or maybe it's a DPST(?), research for your own needs) allowed me to provide the 120v power to these circuits whenever it sensed 120v external power, otherwise all my outlets and fridge and microwave would have been unusable, even when on shore power or the generator. This simple $9 device made things usable again, until the Inverter was repaired/replaced several weeks later
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
110,000+mi since '14 - US, Alaska, Canada (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )b