I can't plan your trip for you and won't try. I don't know when in April or May you'll be here, what the moon phase will be, what locations you'll be at, etc. I will give you some tips though.
April and May are normally dry months, so you probably won't have to worry about flash floods. If you do get a big rain storm, don't get caught in a wash or trying to cross washes or rivers with rushing water. Rain is great for amphibians, especially after dark, but not so great for reptiles. I would wait a day, or so, after a big rain before road cruising. Instead I would herp different habitats during the day.
Walking the habitat is kind of a lost art with a lot of herpers, but it's pretty cool seeing herps in their natural habitat as opposed to seeing them on the road. If road cruising is really a big interest, I'd pick a nice blacktop in good habitat and work it real well for the first hour or two after dark. Road temps usually aren't very high in April and May and your success might depend a lot on luck. I usually feel successful if I see three or more snakes in an evening of road cruising. Lots of times you get skunked, which is another reason not to put all your eggs in one basket. BTW, lots of folks like to walk in the desert with a lantern to look for rattlers. Road cruising can also be good in the cool mornings and even later for coachwhips and the like.
I like herping in the morning, maybe because I see a lot of birds along with a few herps. The temps are changing from cool to warm, as opposed to warm to cool in the evenings. A typical example would be hiking a canyon like Sabino Canyon in Tucson. Starting at the bottom where it's the warmest the canyon starts to warm up and you can catch a few herps coming out to sunbathe, especially the lizards, including gilas. The stream has small fish, frogs, and garter snakes, while there may be a few other reptiles nearby. By the time you've gained a few hundred feet elevation you're probably going to be in the range of more rattlers, like the blacktail and tiger. Diamondbacks are probably throughout the canyon. Another really good herp in the area is the giant whiptail lizard. You might need a telephoto lens to capture a photo, however.
Hope these little notes help some... TC