Along with watching how much water you use for a shower you also need to consider how much water is 'used' when everyone was brushing their teeth, and washing their hands and rinsing dishes.
Many of us are used to letting the water run when brushing teeth at home. Hard habit to break, but when in an RV you can't let it run.
Doesn't sound like you are using that much water but it does.
I had the same problem you did because my sporting events also do not have hookups.
I also stopped washing my hands and brushing my teeth in the bathroom sink because the water from that sink dumps into my black tank. Which was causing my black tank to fill up too fast also. So yes the water used for brushing teeth and washing hands often in an RV does make a difference!