Serial ports shouldn't drain too much they work on the voltage differences and are generally pretty flexible as far as what voltages actually are ones and zeros. I once tried to convert a 0v/5v signal to -2.5v/2.5v and could actually receive it with a laptop, although admittedly there were a lot of bad bits in the reception that I ended up using a max232 anyways.

My guess is that it is possible to drive several receive lines with the output of a single tx line. And if that doesn't work you could use a couple of max233's if you really need the amplification, convert down to 5v, and then hook several of the 5v->12v steps off of that line. It can be done cheaper with max232's which need the additional external caps.