Your internist is advising you well, but very conservatively. Some anti-psychotic agents cause a condition called neuroleptic malignant syndrome (NMS), which looks something like MH but has a different underlying cause.
Nonetheless, the conservative approach is to avoid this class of drugs in MH-affected patients. Phenergan is used for nausea in the hospital; it is unlikely you would need it, but it would be reasonable to avoid this class.
The drugs used for labor analgesia are safe.