Mazda MPV 2000 P0455 EVAP System Leak - Check Engine Light On, Gas Cap Fix, Cost Comparison & Serpentine Belt Concern

My Mazda MPV 2000 has been showing error code P045- EVAP Emission Control System Leak (Large) since August. The check engine light remains on consistently whenever I take it to my neighbor for diagnostics. We previously cleared the error and replaced the gas cap—ensuring it was securely tightened—but about a week later, the check engine light returned. The vehicle is currently running fine, but as a military family with four young children, I'm concerned about potential breakdowns during our upcoming 250-mile move north. My van has approximately 78,000 miles on it. I’m trying to decide whether to take it to the dealership or a local independent service station for repair—what would be more cost-effective? Additionally, my husband has no car knowledge—I handle all research and maintenance. I’ve also been told that the serpentine belt shows signs of cracking. I hate driving with the check engine light on, but budget constraints are real. Should I prioritize fixing this issue before our move Friday? Any advice on next steps or potential causes? Note: The catalytic converter was replaced by Mazda about two months ago—could this be related to the current P0455 code? If not, is it worth returning to the dealership for repair?