It's been a number of years since my canyon tour , by car , did both routes , I wouldn't consider either straight or flat . Do able in an RV, but not straight or flat.
Southern Utah, and northern AZ are high elevation areas and the roads should be considered mountain roads.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.