I got a reply from Parallels Support. They recommend disabling Travel Mode entirely.
"Please try to disable Travel Mode using either steps:
- On the Mac menu bar, click on the Parallels Desktop icon > Action > Quit Travel Mode.
- On the virtual machine's configuration > Options > Travel Mode, set Enter automatically to Never.
Afterwards, please check the Resource Usage slider abd make sure it is set to No Limit. Test and observe the virtual machine if the slider still reverts back to the lowest state."
Have only tried it a day or two but it seems to be working.