Commercial OTR trucks are always level. Our Mil trucks were made to be tilted during offroading (some as high as 45 degree), and maybe even parked at large angle of tilt. In those cases the lower tank can overflow, and the upper tank starve for fuel. For those reasons the crossover might not be a good idea.
See previous post I made. Two vehicles I can think of, mainly two I've driven in the Army that have cross a cross over line on the bottom of the tanks. Oh, and the kicker is the tanks aren't the same size.