Hey tunes, If it were me, I would I would take I-10 to Blythe and then take US 95 to Laughlin or I-10 to exit 192 and take route 177 to route 62 to US 95 and up to Laughlin. But, then I like to go the less traveled way. The shortest route is I-15 to I-40 and over to US 95.
Depends on what type RV you have. Pulling up from San Bernardino to the High Desert at Victorville is a pretty good pull on I-15. I have taken that route with a "95" Dodge Cummins diesel towing my HitchHiker at 30 mph with my emergency flashers on. I could take the same little ol hill with my "03" Dodge Cummins at 70/75 mph. So it depends on what year and type RV you are talking about. I traveled from Laughlin to LA on US 95 to I-10. The only hill I remember is out of Laughlin on route 163 connecting to US 95 South. It is a pretty steep pull for about 7/8 miles. That can be avoided by going to or leaving Laughlin through Bullhead City, AZ via AZ 95. That route connects with I-40 at Needles CA. Hope that helps.