Travelling to Rishiri via Heartland Ferry

Hi I have a camper van around 5m long, two adults and two children. I understood from the Heartland Ferry webpage that the driver's ticket is inclusive of the vehicle's fee. I have the following questions.

1. Is there a discount for round-trips ?
2. Is there a discount for children ?

I'm really keen to visit Rishiri to drive around, hike a little and cycle, and if bringing the camper van over to Rishiri is too expensive, what are the cheaper options if I have if I were to planned 3 days 2nights there ?

1. Is there a discount for round-trips ?

It seems that there is a 10% discount for cars only, but not for passengers.

2. Is there a discount for children ?

Elementary school children (6-12 years) pay half. Below 6 they travel for free.

if bringing the camper van over to Rishiri is too expensive, what are the cheaper options if I have if I were to planned 3 days 2nights there ?

You could park your camper in Wakkanai and try to navigate Rishiri without a car. Rishiri is one of the most expensive places in Japan to rent a car. The cost for a 3-day rental would compete with bringing the camper over.