Take a Breaker with you to Home Depot or Lowes or _____ hardware or Electrical supply.
As others have tried to say and I think you are planing on doing
Main breaker box-(30 amp branch)----Inverter----Sub panel (branches to loads) sub panel may be part of inverter.. or not, since yours is after market I'd guess NOT.
Suggestion: If you have not already purchased the sub panel.... Get one that takes the SAME TYPE OF BREAKER that way you don't need to worry about "This one or that one". That is one thing CW did right on my RV. (They made two mistakes on the install but they got that right).