Couple of thoughts here.
Newmanstown to, say, Phoenix is about 2300 miles. Out and back is 4600 miles. Three weeks is 21 days. That is about 220 miles driving EACH DAY. IMHO opinion you will not be seeing much except the inside of a windshield. Certainly not be able to truly see things; just say you have been there. Reconsider time element.
And I would avoid the Hwy 550 mentioned above unless you are an experienced mountain driver and, even then, with the tv or toad and not the rig.
Would recommend you buy Mountain Directory West for info.
Just my thoughts.