A slum landlord allows his property to devalue, because he won’t invest in his property. I don’t know if Fred Trump qualifies as that. Being a low rental does not qualify one as a slum landlord.
I think he was a savvy businessman and when I used the word “slumlord” then it was to describe the society’s perception of his business of running lower income residential housing, even if done competently, being low prestige.