The full RMP wave goes straight to zero after the ‘attack’. I’m not sure what kind of sound you’re going for but it’s not much of a decay! Anyway, that’s beside the point. Have you tried other LFO waveforms, like TRI in half mode (which looks like equal attack and decay)?
What are your other settings? I would assume having the half TRI wave on the LFO would allow a crude attack/decay if your attack and hold in the amplitude page were set to zero, and decay set to maximum. I really need to start reading this thread while I’m actually in front of my monomachine haha.