I had one from Halfords, i bought it when we went to Blackpool 3 years ago, the fuel consumption wasnt that bad TBH, from london to leads then to blackpool then back to leads then back to london (this includes going around as well) it was 3 days and my total fuel consumption was 2 1/2 tank. all 7 seats was occupied (4 adult, 2 children and 1 baby), the rack was full loaded as well. average speed is 70mph.
surprisingly the bus was running soooo smooth, even with the heavy load. probably smoother when its empty. the racak was aerodynamicly shaped anyway so i dont think its creating that much drag, much of the extra fuel consumpsion is probably from the heavy load specialy on the gradient roads.