I have used the crayola ultra washable markers for some time. I use them pretty heavy too. I know I have left them on for a few weeks.
I can say that I do pretreat before I wash. I dilute dish soap (Dawn works but I have also used Ajax just fine) I water it down like 50/50 or 30/70 (soap to water) then I spray it all over. I let it sit for a few minutes.. Knock on wood everything has washed out fine