How to add small 220 Volt Heater from an existing 220V 30 Amp receptacle
The best way to tackle this issue is to remove the 30 amp plug and replace it with a junction box. Locate a sub-breaker box on the wall and feed it with 10-ga 2-wire +ground cable from the new junction box. With the sub-box you can create two or 3 circuits with different amp loads (15, 20, 30) and wire sizes. Be sure to enclose the feed and any load cables in conduit if they are exposed to the living area.  And follow all safety rules, including shutting off the breaker at the main box while the work is ongoing.