So, to sum up: Trump has offered scant details about how he would replace ObamaCare. But what little he has said is philisophically consistent with the arguments in favor of single-payer, a policy approach that he has praised in the past.
The whole irony of this is that right now, Sanders and Hillary Clinton are in the midst of a heated debate in which Sanders is arguing in favor of single-payer and Clinton is saying it would go too far to be politically feasible.
Should Clinton and Trump be the nominees, it will have meant that Democratic voters will have rejected the candidate pushing single-payer health care and Republicans will have embraced him.