If the slipping only happens on a 90 degree turn, could your problem br in the rear end?

Are right and left turns the same?

Does this happen when cold and warm?

A mass change of fluid can cause issues - many do a slow fluid change, drop the fluid in the pan and top up - this is what the 740 service calls for every 30K ( I am not sure of the 30K figure)

Speed the slow change by changing the pan fluid every oil change.

For fire trucks with automatic transmissions -

1 - Drive several miles to warm up transmission fluid
2 - put transmission into every gear for 15 seconds or so
3 - check fluid level at 1000 RPM in neutral