First time running on trailer at idle and I have suction through the dipstick tube.
It's a 2004 350 Mag.
Seems to idle okay but what is going on here?

Might be just the PVC valve if it has one. Have a serial number?

Ow180973

Pulled out pcv and both seem fine.

The PCV system has to be open on the other end or it will creat vac in crankcase. How is other valve cover vented? If PCV system is ok seems it would have to be intake gasket.

hmmm,,,running a vacum pump???

The serial number is for the drive
If its only a slight suction then their may not be an issue. The PVC is constantly drawing air out of the block. Normally there is one PVC and the other side is just a vent which allows air to be drawn in or let excess out it there is to much blow by. When the dipstick is removed it maybe an easier path for the PVC to pull air, and if so this would be the reason.