Question about septic tanks

I am looking at buying my second park. this park has septic tanks which i am told need occasional pumping.

What causes a septic tank to need pumping? poor drainage? abuse by tenants flushing everything down?

How much can that cost to pump a tank?

If people are lot renters and own their own unit, i would assume that septic pumping is still the responsibility of the park owner to pay for, correct?

thanks for help