Should I tie into the ERV or just vent outside to increase bathroom ventilation?
Firstly check if the filters on the ERV unit need replacing, as dirty filters will reduce air flow.  (Our filters need to be replaced every year and cleaned a few times a year.)

Your ducts may be blocked somewhere.

The outlet from the unit, to the outside may be blocked.

It is likely that the inlet in the bathroom can be adjusted to allow more air flows through it.  However this will mean less air gets removed from other “wet” rooms. On our this is done:


Turner the cover to remove it from the ceiling. 
Undoing a locking nut
Turning the value head to increase/decrease the size of the gap.


Most ERV units have some way of controlling the speed of the fan, but turning it up will use more power.

Some ERV have a boost function that can be control from a switch or sensor in the bathroom room. 

However to do a good job of this you need an air flow meter to check what is going on.  It will help if you know if the ERV ever worked well, as there may be a design fault or it may not have been installed according to the plans.