Why does the breaker in the inverter trip instead of the breaker in the service panel.. Simple.
Panel--30 amp breaker---_Inverter---15 amp breaker---20 amp load
This is what you get for having a cheap inverter install (NOTE: I am referring only to the install, not to the inverter here).
My coach
30 amp breaker--Inverter---Sub panel--15 amp breakers INSIDE THE RV---Loads.
More expensive but I do not have to go out in the rain and reset.
As for the comment on the "Cheep inverter install" My inverter, since it does not contain the sub panel, is actually cheaper than yours, but the install is more expensive (Due to the added box needed).