At Goboony you determine your rates yourself.
To get an idea of the market prices you can look at the site for comparable campers that are often rented out. Preferably start with determining your high season rate (July and August) and charge around 60-70% of this rate for the mid season (June and September) and around 50-55% for the low season (the rest of the year).
As a starting Advertiser, it's best to opt for a slightly lower rate than the comparable motorhomes, since Travellers will compare your ad with previously reviewed ads. Once you have received a few reviews, you could start increasing your price step by step.