Saving on holding tank pumping

I have a black water holding tank and put zero paper in the toilet and use a ceramic boat tankless toilet. Difference in emptying costs for one person, 1750 gallon tank is almost 2 years vs 2 months. Emptying costs vary but are around 350$ Cdn.  Otherwise 12 months at 350$ per shot 4500$ /2years vs 700$/2years.
Not bad.  

I dont quite understand what you have and what you are emptying?
What are the 2 years vs 2 months about?

We had a place where we had a holding tank.  We only paid to have it pumped when it was needed.

Seems to me we would only have it pumped every year or two.

So what is the money saving tip?

Zero paper in the toilet and use a ceramic boat tankless toilet?  Seems like with this the tank would never need to be pumped.