In the U.S., it has to be southern California, right on the coast. But that's going to be humid. Get more than a few miles inland, particularly over the first range of hills, and it can get over 100 F for days at a time in the inland valleys.
The kind of place you seek exists, but not in North America, and not where you might RV. But you don't have to move, it is always springtime.
The environment is high altitude inland mountains in the tropics. Highlands in Southeast Asia, some mountain areas in southern China (Shangri La is real) and some mountains in Equitorial Africa.
My favorite is Kunming, which has average temperatures from 8 C to 20 C year round, though the highs can reach 25 and the winter lows withing a few degrees of freezing. Winters cool and dry, summers hot and wet. The monsoons thus might be a problem for you, as they can be anywhere in South Asia.
Vientiane might work better for you, with more even temperatures year-round, averages consistently close to the highs. You would be looking at 7-8 (rainy) months staying just under 30 C, nights cooling to 25 C, and temperatures 16-18 C during the short dry winters.
But you said 75 F, not 85-90 F, so we need to find something with that tropical temperature stability at a higher altitude. It will likely be a remote highlands village, so living there would be like camping, as well.
Cities, and city amenities, are rare in these places because commerce in the tropics has long depended on waterways for travel, so most development is along the big rivers, with the important cities being seaports near major river deltas (Saigon, Hanoi, Bangkok, Guangdong, Manila, etc). The mountains were always a major trek, still difficult today with rail and air travel.
