Ever driven one? ridden one? seen one? The jingle tickled my Mother; she used to ‘sing’ it, ‘…to you at once direct from Madman Muntz’. Father was too economical to rent one. I remember the little cartoon Napoleon character in his ads.
Sorry, never heard of them. I remember Muntz TV’s though. 
I never lived in LA and would not have heard of him. Did you live there in the 40s and 50s? Here’s a story about him from the LA Times.
I lived in San Francisco in the '50s. I think he was state-wide, because I saw his print ads; there was always a little Napoleon cartoon character, left hand in his coat, right hand extended up. I heard or read an interview with him years later. He said the worst renters were dentists, they didn’t deal with even the smallest problem themselves. I thought I read that the Motor Mansions were his first product, but it seems otherwise. I see he made his own cars, Muntz Jets for a while. That’s a good way to lose money.
Can’t find any pic of a Muntz motor mansion, can you?
Near the end of the documentary about him there were a few shots of the motor mansion, from what I can find out these were production RV’s that Muntz bought elsewhere and brought to a lot in Los Angeles to resell.
Now that’s an interesting guy.