I have the same sled, and yes , with a tank filled to the top, the motion of the sled down the trail will bleed off a small amount of fuel from the breather tube, as you mention. There were threads about this long ago, no time to search now.
Look at your sled from between the top of the tank right by the steering column, there is a clear plastic tube, about 1/4 inch dia.
Follow it to the left where its strapped to the "firewall yoke". Dunno what to call it, LOL.
It just comes to an end, about 1 ft. above the chassis. Cut a couple ties, and route the end upward, careful not to kink it.
Thats what I remember about it. Hope it helps.
There is a vent line coming from the top of the tank and follows the frame and ends just above the running board, therefore if gas does go out the line in ends up on the running board. Thats how my 04 was.
What I did was rerouted the line so it runs higher than the tank and less likely to drain fuel, then I extended the line down thru a hole in the running board, so if it does drain it doesnt end up on the machine.
It will only drain if the tank is right full of fuel, or if you hit some nasties and the fuel splashes into the line. I figured if the line runs higher than the tank any fuel that gets into it will hopefully drain back into the tank.
I had the same problem on my 05 t660 and it turned out to the vent line was pinched near the reverse handle. Thus the tank would pressurize and you would smell fumes coming from the gas tank itself. The tank when pressurized would become more round and you could look under the tank and see light coming from the other side. Once the the line was rerouted that was the end of the smell. Also the smell would always come after riding for a while. Another way to see if that is your problem, is when you have the smell remove the gas cap and if it was under pressure you will hear the air escape.
The important thing is to make sure the line will freely drain, no low spots where condensation can sit and freeze, and keep it away from the exhaust system or any source of heat. Dont want to take a chance on raw gas to catch fire, not good as you can imagine. You can see mine running along the steel tube and down to the foot well. I'm sure there is a hole there to drain that area that I put it thru.