If you are planning on taking I-15 through SLC, then take it south to Spanish Forks where you pick up US-6 east to I-70. Take I-70 east to US-191 and then south to Moab. When you head south to Sedona, continue on US-191 to US-160 at Mexican Waters, AZ and then west on US-160 to US-89. South on US-89 & US-89A to Sedona. All good and direct roads. We have traveled every inch of that route and it is very doable in any RV.
Personally I despise I-15 through SLC, but for your starting point, that is the best route. We usually cut down through Ely NV on US-93 then pick up US-50 east to Scipio and Salina UT, where we pick up I-70 and head east. Longer, but keeps me out of the SLC mess and IMO, is more scenic.