If I have absolute control I would elect to have all my content posted on the root domain (dealersite.com). But when it comes to content marketing and integrating with template dealer sites, the best decision is to create a sub domain (blog.dealersite.com). It's very easy to allow the content to "travel" with the root domain, no matter which template/company you are using. All it takes is updating the DNS.
We use this strategy very effectively with all our digital clients, for this very reason. Once you pay for search optimization and content marketing you should own the content.
Additionally, if you do decide to use another provider, it's recommended and really easy to either create your own, or get the provider to send you an html file for every custom page they build for you.
Craig - Respectfully, I'll disagree with you on your opinion of WordPress 
- I think you could make similar complaints about any content management system. There is no perfect solution, but we find WordPress to work very well for us.
Nick - We have built custom dealer websites using WordPress, and would be happy to help you in any way I could. Send me a DM and I'll show you some examples, tell you about some hurdles, maybe help point you in the right direction.
