A significant number of inland paisanos (natives) migrate to the coast for Semana Santa, the roads going East to West and vicesevrse will be congested at the beginning and end of the "vacation", some roads going North and South will also see an increased volume of traffic, be very careful on the roads and have patience.
I wouldn't call it bad but it will be very busy. Lots of Paisanos getting TIP's and FMM's. The lot @ 21 will be very full and the line ups at the Banjercito will be long. The Aduaneros are on high alert for "marks" who will pay mordida. However there will be extra staff to handle the crowds. Be very alert on Highway 15. Lots of traffic and most drive day and night!. I have done it but had an FM3 and was in the no hassle zone so did not have to stop for a TIP. I think the first checkpoint just past the toll booth will be the worst area for an RV. There is not enough room for all the cars to park and go inside to pay duty. Look upon this as an experience! BTW: The middle of the week is not the best time to cross. The paisanos that are going to Sinaloa will be crossing then as they want to be home before Friday. Enjoy.