Here goes...
1. You need to pull the roof and clean the evaporator it's probably loaded with crap. I pull my roof every 2 years and clean the ac components up top and while the roof is off (the 8 longer screws hold the roof to the top of the rops structure, the rest (shorter ones) retain the headliner. Even though the side mount filter traps most of the dirt, some will still get by and clog the evaporator, causing loss of cooling power. Easiest way to tell if the evap needs cleaned is by smell. If the conditioned air has an odor to it. the evap and probably the heater core is loaded with crap and needs washed out and rinsed. While you are doing that, make sure the drain hoses aren't blocked as well.
I suggest lining the underside of the roof with foil-bubble-foil Reflectix. The Kubota roof is two pieces molded together but has no insulation whatsoever, so the sum beating down on the roof really heats the air space between the roof and the ac/heater blower intake. Adding Reflectix really cuts down on heat transfer and allows the ac to work better. You can cut the Reflectix to size and glue it on with 3M contact adhesive.
2. You need to direct the airflow better through the condenser under the hood. I had a 5030 years ago and I added rubber mudflap strips to each side of the space between the condenser and the inner frame parts. You can also add a 12 volt automotive rad fan and wire it with a relay to the compressor clutch to come on when the ac compressor is energized (What I did with my M9000 instead of the rubber mudflap air directors). Airflow through the condenser is paramount. Little airflow means little heat transfer and little heat transfer equals poor ac function.
3. Depending on the age of the tractor, your refrigerant level could be low and need topped up. If you aren't familiar with charging an ac system, I suggest you taking it to a shop that can do it, but all systems leak some. Some more than others but they all loose refrigerant over time. Easiest way to tell if the refrigerant is low is look at the sight glass on top of the receiver dryer with the ac running on high for a while. If bubbles appear you need to add refrigerant. If there is no liquid movement in the sight glass, you need refrigerant. If there is liquid flow with minimal bubbles, you need to clean the roof evaporator, check the roof sealing gasket. add Reflectix and a new intake filter as well.
The output temp on my M9 on max air is about 36 degrees and it will freeze you out, sunny day or not and my windows have only the factory tint.
Like I said, I clean my evap every other year. Cleaning frequency will depend on your end use but they all need cleaned eventually. If it smells, you need to clean it. If the airflow is lees than a windstorm, you need to clean it as well. Not a hard job, just tedious.
How I do it. My M9 is a 2004 btw. It takes R134. Not sure about your 5030. You need to check with your dealer but if it has R11 or 12 and it's low, you'll need to have a qualified ac service center (or your dealer do a refit as 134 isn't compatible with 11 or 12).