Sweat all the way back to the compressor, after the unit has run for a time, getting the ambient (room/cab) temp in perspective is an indication that your freon level is adequate. Also if you have a sight glass on top of your expansion chamber/reservoir being clear with no bubbles and having the sweat back means you are full of freon. Course sweatback is temp humidity related and on hot dry days you won't see the same thing as cool wet days.....I mean you are condensing water vapor from the air on a cool surface.
30 degrees of temp difference between the two for both the evaporator (30 degrees cooler) and the condenser (30 degrees higher) is what I was taught in school. The system has a capacity that rides with the tide of the ambient conditions within the limits of the unit.....aka if the unit is overloaded by heat input or low/high freon it can't respond correctly. Normally once the system stabilizes and is not over taxed and the thermostat is in the coldest setting, it will maintain the 30 degree separation between input and output and continue to drop the ambient (room/cab) which drops the input and the output and etc. until it reaches it's cool limit and the stat shuts off the compressor. The stat doesn't change the cooling rate; the capacity of the system does that and if the compressor is engaged it has one rate. All the stat does is determine how long it will run at it's fixed rate.
Fan speed has a lot to do with it and if not high enough can allow icing and blockage on cooler days and once it starts icing it just gets worse. However, in a tractor cab in the summer that shouldn't be a problem. If it is, you have a problem with the system, usually over charged with freon which also drives up your pressures and can blow a seal in the system among other things.
One thing about cabs is the fact that engined/drive train heat and radiant heat from the sun, can be a big factor in cooling a cab. My rule of thumb for an average sized tractor, like my 65 hp is a 14,000 BIU's. A good size cooler for a 700 sq ft room in your home which is well insulated and most of the sun's rays are blocked, but for a tractor cab with 360 degree visibility, it can be a test by the environment for adequacy.
Agree that you check your heater valve and ensure it's closed.
Just because the tractor is new doesn't mean that your AC fill was adequate or that you don't have a leak somewhere, or that it wasn't over filled at the factory. Sounds to me like you have something wrong and as you said, now is the time to get it fixed.
You can do some checking on your own and might look up "AC wet and dry bulb measurements" on the www and get an idea for yourself as to how to test and what to expect.
Mark