It would be my pleasure to help you explore all available options for making QuickBooks Online (QBO) work like your business needs. Do you mind explaining a bit more about how this fleet management software would operate? Having this information will enable me to make the best possible suggestion for your situation.
In the meantime, QBO does offer the support of thousands of various third-party applications for any functionality you may need. To see if there's an app that can help you accomplish this, head to the Intuit App center and type a keyword like Fleet in the search-bar.
Please keep me posted here on how I can help you succeed with QuickBooks. Thanks for reaching out, I'll be standing by for your reply.
I'd be glad to point you in the right direction for linking your QT Fleetmaster Credit Cards with QuickBooks Accountant.
There's a community dedicated to developers and linking of applications with QuickBooks. This site contains resources for software development, as well as the ability to ask fellow developers questions. You can find additional information about this at the following link: Intuit Developer.