It sounds like you plan a day trip to Hoover Dam before going to Las Vegas.
Bad idea.
To visit Hoover Dam you have to go through the security checkpoint on NV-172. The ONLY way to get to where you can visit the dam is exiting US-93 at Exit 2 in Nevada.
You will have to drive down the steep twisty grade, drive across the dam with a couple hundred people walking inches from your RV, up another grade and park on the Arizona side of the dam.
There is no access to the RV parking areas from the highway in Arizona. Only by driving across the top of the dam. The old US-93 route on the AZ side is blocked.
Shortly after the security checkpoint is a parking lot for the new bridge access area. A very good exhibit on building the new bridge, and there is a walkway along the new bridge with a fantastic view of the dam.
It is almost 200 miles to the dam from Zion via I-15 -> I-515 -> US-93. That is going to take more than half the day. A two or three hour visit to the dam, and you will be getting to your Las Vegas campground very late.
Taking NV-169/ 167/ 147/ 166 along the Lake Mead west side is not a shorter distance, but is much slower due to the 45 mph speed zone in the National Recreation Area.
We made the Zion to Vegas trip last Nov. Frankly, the very best part of the Vegas area was two days we spent at Valley of Fires State Park northeast of Las Vegas. A must see stop in my opinion.
Also the drive through the Virgin River Canyon on I-15 where it cuts across the corner of Arizona is great. Wish we had been able to spend a night or two at BLM campground in the canyon.
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