Shared Hosting Multi-Domain Setup Dedicated IP

I have hosting with Hostgator and a dedicated IP. The root domain is working fine and I can create sites and they all work great. I want to offer other domain names as part of the service I am providing however I don't know how to make this happen with shared hosting. I have tried to follow the addon domain method of making this happen and Apache spits the dummy causing the dedicated ip to be a shared ip and routing data in the wrong directions.

I thought I might be able to do this with parked domaining instead but am unsure how to set this up beyond adding the parked domain. The parked domain I have added is resolving to the root site and the wp-signup.php page comes up as expected however I can't create sites with the non root domain I have added. I am sure I have to add in a wildcard somewhere to make that happen but am unsure how to do this with a parked domain.

Have I gone about this the wrong way entirely?

Thanks for all and any help.