Best route from Merced Ca to Mesa Az

Hwy 58 ends where it meets I-15 at Barstow, from I-15 take I-40 (the Needles freway) to Needles.

TexasShadow wrote:
we use the 58 to needles, 95 down to I-10 and straight on to PHX. They have turned 60 into a pretty good traffic carrier east of I-10.
If you just want an overnight parking place, the junction at 58 and 395 has a big parking lot just east of the light on the right. we've stayed there, but it's busy with trucks.. and we've stayed at Barstow at the Flying J which is a mile south of the I 15 and 58 junction.

This.

we use the 58 to needles, 95 down to I-10 and straight on to PHX. They have turned 60 into a pretty good traffic carrier east of I-10.
If you just want an overnight parking place, the junction at 58 and 395 has a big parking lot just east of the light on the right. we've stayed there, but it's busy with trucks.. and we've stayed at Barstow at the Flying J which is a mile south of the I 15 and 58 junction.

Ditto the Lancaster (Antelope Valley) fairground if you go that way.

I mis-spoke when I said I overnight in Tehachapi. Driving motorhomes for the company we have to sleep in hotels and Tehachapi fits the bill. I don't have any info on RV parks, sorry.

Lancaster fairground. Or leave early 5, 210 (mid day only), 10 and stop east of LA.

I assume you are looking at the route through Palmdale that takes Hwy 138 to connect to I-10. That route will involve more southern CA traffic. I would take the route suggested by azdryheat. I've drive 58 and I-40 many times. If you just want a quick easy stop for the night, I've stayed at Newberry Mountain RV Park just off I-40 about 20 miles east of Barstow.

I do sorta that quite often; Tucson to Sacramento delivering motorhomes. 99 south to Bakersfield. 58 east through Barstow to I-40 east to Needles then south on the 95 to I-10 to Phoenix area. Easy peasy. I, and our other drivers, have found this is the best route as we've tried everything else. I always overnight in Tehachapi.