HI Doug,
3rd Party App:
There are a couple way to proceed. I do the accounting for a local Used car dealer and for that account we use a software application called Dealercenter. Perhaps that could help solve the problem by keeping the information that you needed classes for previously within the 3rd party app and only pulling in the data necessary within QuickBooks Online
Using Projects with in QuickBooks Online:
Another option within QuickBooks Online would be to use the Projects feature. You would need to create a new project per vehicle, but this method will bypass the need for an ever growing list of classes or Accounts and Sub Accounts.
Upgrade to QuickBooks Online Advanced:
The recent limit levels have impacted our accounting firm as well and has required me to redesign our workflow for many of our clients, but not all of them. After playing with QuickBooks Online Advanced, especially some of the new features coming soon, I know we have a good segment of clients who would benefit from the upgrade while also allowing us to save time at least an hour per month per client just by using the new custom fields that will be released soon.
In your situation, the new custom fields could really help you to track sales by employee in a fraction of the time!
Hope these suggestions helped out!!