The best rt from vegas to dallas ..not I10 and not to high mountains ~!?

Which rt can I drive without having to climb big mountain with my old Van??!! I need to drive from Las Vegas to Dallas Tx my Van is old coming here i drove on I 10 and of course at night so it would not be too hot but there is still the hoover dam. II remember 30 years ago I drove 1 time from vegas to dallas and drove over utah colorado and than down to tx. Please when you know an good rt I can drive I thank you. :) Please help