Best route question.

There are a couple different ways to go. But we jump off I-10 at Quartzite, AZ. onto US.95 N to Sin City. This is in our dually crew cab with a 39' fiver. No problem. Just buy fuel before or after Needles, CA. Usually the highest priced fuel in the US.

From I-10 at Phoenix take US 93/60 northwest to Las Vegas. You will be on I-40 east of Kingman to junction I-40 US 93

93/60/I-40/93 is a good route. you will cross the high bridge over the Colorado river. They put you in the inside lane for that so your passenger doesn't have to look down about 800 feet. ha ha.
To get to 60/93 from I 10, take 303 north to 60. That's on the west side of Phx, about exit 124.
Up the road, near Wikiup, there is a nice little fed campground called Burro Creek, with water. On the creek, nice and quiet.

I10 to 17N to 303 to 93N. Less traffic than the 60 and dumps you out by Lake Pleasant. Then though Kingman on the 40

Hoping to go from Maine to Florida next August to September and want to see some major cities as well. Pulling a small car behind 30 foot motorhome. What is the best route? Do I need to reserve campsites? and how long should I allow for the trip?

Explorer2:
You should start your own new thread.
It depends on what you want to see and do. We avoid interstate highways. We have done the Smokey Mountain parkway. We have done US 1 along the coast.
Each road has it's benefits and negatives. We take day trips in car to hit the specialty locations.