I am not affiliated with BJP, RSS or any of their groups. I am merely a supporter of the current Government. This is my opinion about their views on Sardar Patel.
Vallabhbhai Patel was one of the most prominent freedom fighters who brought us Independence. When India became Independent, he was the first Deputy Prime Minister, first Minister of Home Affairs and the first Commander in chief of the Armed Forces (Now only the President has this title).
He was also the man who United the nation by bringing many princely states under the rule of India. This endeavour got him the moniker "Iron Man of India". Sardar Patel was a selfless leader who kept the interests of the nation above all personal bondages and interests.
This kind of personality matches with the kind of ideology RSS pushes forward. It is also something the top brass of BJP endorses. So they support Sardar Patel.
Also, one doesn't need to think about Sardar Patel's views on RSS in this situation. That's past and gone. One doesn't need particular reasons to honour a national hero and that's probably why the RSS supports Sardar Vallabhbhai Patel so much.