I'm not sure of your rig, but I think you will find that driving 600 miles in 8 hours is unreasonable. That calls for AVERAGiNG 75 miles an hour. Given that you are likely going to have to make at least 1, and more likely 3 or 4 pit stops for gas, potty, eat, etc., there is legally no possible way to accomplish that.
If you managed to do it with a single stop for 20 minutes, without ever encountering traffic, you'd have to drive a tad over 78 miles an hour. If you needed 3 such stops, again without encountering anything else to slow you down, you would need to be driving 85.7 mph.
My own experience has shown that going 60-65 mph for safety while towing on the interstates yields an overall average of just over 50 for the day. Your mileage may vary, but I suggest you consider cutting back a bit on your goal here.
Oregon to Maryland and back, is doable in 3 weeks, but not if you plan to make any stops whatever that are not essential, and you will end the drive totally exhausted. There is no way I would be able to keep up a pace like that for even 1 week, so you'd have proven yourselves better men than me.