The library comes with examples (just scroll down or hit the GitHub icon) and the pins to use are Vin, GND, RX, TX.
One odd thing with the examples is that an immediate build seems to fail.
After a short while or a minor change (e.g. #include "Nextion.h" to #include <Nextion.h>) building usually works - no clue why that is.
Unfortunately this early release version of the lib doesn’t allow for easy use of these, but I’ll implement a workaround soon.
My original endeavour to wrap this in a proper C++ library got stalled by other things, but with that you should be able to even use ParticleSoftSerial.
Meanwhile you may be able to move your max3232 to one of the alternative interfaces or have to wait for my workaround.