power consuption of internal 32kHz RC oscillator
Can I expect the RC oscillator to draw even less current than crystal one?


You can expect nothing.  However, a guideline to reading datasheets that I rather disdained when I heard it from an old guy with bushy white eyebrows 35 years ago, and which I've come to believe solidly since, is that if the parameter isn't mentioned in the datasheet, it's because the manufacturer isn't proud of it.  If the number were a good one, it'd be in there.

It could be an oversight -- I'm using a couple of processors from that family at work, and the manuals are woefully inadequate compared to what ST provides.  But it could also be that the numbers just aren't stellar.

If you have an important spec to meet (and "the same or better than the DS1337" is far better formed and easy to pin down than many I've seen) then you should choose your hardware based on that spec, and don't proceed unless the datasheet shows you how, with margin.  Otherwise you're just rolling the dice, and the dice are weighted against you.