I've had my RTV for a couple years now, a 2006 camo'd Worksite model. One of the first things I did to it was replace the Craptyle 489 tires with ITP Blackwater Evolutions just like Ovrszed did.
I believe the standard tires on a RTV (at least my model) is 25-10-12. I got all four tires in 26-12-12 so I could use the same wheels. Given how well it fits on my RTV, I believe I could have gone to 27-12-12. I got mine from AllTerrain Online out of TX; Robin the owner was very helpful. Excellent prices and excellent service. I had them shipped to my local tire company and had them install them.
ATV Tires, Wheels, Winches & UTV Accessories - All Terrain Online
I live on a hill, both front and back of my property. I don't believe my 4WD works on my RTV, so I'm running around always in 2WD. I'll take my RTV in 2WD I won't take my Kubota
L4330 in 4WD. Having said that, I don't typically take my RTV (or tractor) up and down my hills when it's wet and I don't have mud to deal with.
ETA: If you replace your tires on your RTV, do not use ATV-type constructed tires, they are not heavy-duty enough IMO. For the heavy RTV, you really need a 6- or 8-ply tire for it. The ITP BW Evolutions are 8 ply and have one of the heaviest weight ratings I could find.
Pics of my RTV and it's tires, note the hill in the background, I own it all the way to the bottom.