Even though you have a very short rig, I agree that taking the toad would be a good idea.
If you are that worried about grades or narrow roads, etc., detour around them and take the toad to see the sights ...I'm specifically referring to Oatman, as mentioned. Oatman is definitely worth seeing. However, it may be better to find an RV park in the Kingman area and then take a side trip to Oatman with the toad.
You may be able to do this in other parts of the Historic Route66 if you feel uncomfortable taking the motorhome on narrow or steep terrain.
Doing it in this manner, you can stay on the interstates when you're towing but use the toad to take side trips on the historic parts of Route66 when you feel the need.
So, I agree, take the Soul --just leave those cute hamsters at home.