Parking brake OFF?
HST in 'H' not 'L' position? (LOW is really LOW on an RTV500)
I suspect old or contaminated fuel may be your problem. I would dispose of existing fuel, remove and clean fuel tank thoroughly and replace fuel filter. If you have bad luck, fuel pump may also need replacement. (People top RTV500 fuel tanks with diesel quite often, without realizing their error.)
If you will expand on your lack of power symptoms through the EDIT function, we can be of more help. My above suggestion is more-or-less a shot in the dark, but based on experience.
Did you receive an Owner's Manual with the vehicle? Have you read the Owner's Manual?