In general after 30+ years of camping in Texas, we prefer to not plan on extensive trips between May and October.
Now that we are full-timers, we don't have the flexibility to stay home. The TT is home.
Depending upon where you are - most of the state will see temps hitting the 90s by early May, and 90s and higher temps into October are very common. There will be occasional cool weeks as a front comes through with temps dipping into the 80s at any time. But you cannot plan in advance as to when those will be.
Daytime temps into the high 90s and 100s can occur any time between early June and mid-September.
The real rough days are those where the LOW temp at night is 80 degrees or above.
The coastal ranges and east Texas might not get that hot, but the humidity will be higher and the muggy feeling isn't pleasant.
In 2016 we spent the entire summer in Texas, mostly in the Hill country, but some weeks around Dallas and others in East Texas and north of Houston.
# of 90+ high temp days we saw were
Apr - 3
May - 9
Jun - 22
Jul - 31
Aug - 22
Sep - 26
Oct - 6
100+ days
Jun - 1
Jul - 9
Aug - 13
Sep - 3
But remember that's one recent year, the next year could be vastly different.
Personally, we find anytime from the second week of October until mid-April to be great camping weather in most of Texas. During the colder months, getting south of I-10, or at least close to I-10 provides nice weather. Not shorts and no shirt weather, but comfortable for being outside.
My method to winterize my rig is to head south of I-10.