I travel frequently with guitars, often pretty expensive ones. (I'll be taking a trip mid-July with six to eight guitars in an area where the heat and humidity is often pretty extreme.) I have
never... almost never... had a problem with heat.
One thing is that I have air-conditioning. While I'm travelling the car is rarely off for more than a few hours, but I have left them in the better part of a day. I think it's also imortant to mention I have pretty good cases for them. Not the ultra-Xtreme super-expensive it'll-take-a-direct-20-megaton-hit kind (although I do have two of those, one acoustic and one electric) (they're also disgruntled baggage-handler proof!) (except maybe in Amsterdam

). Anyway, I have good hard-shell well padded cases. I think that once the interior assumes a given temperature (like the temp of an air-conditioned car or van) that they will stay that way inside for quite awhile. Several hours or more, and once the car/air-conditioning is back on will quickly stop warming up toward the interior. I also will lower the string-tension a bit, at least the six-string, when I put them in the case.
I use a little common sense of course. I wouldn't leave my car or van parked in direct sunlight for six or eight hours straight on a ninety-seven degree (F) day. (Unless I went back to it and started it up and let the air-conditioning cool it down again every couple of hours.)
And when you take your guitar from one temp into another, like from a warm car into a cool air-conditioned hotel room, let it sit unopened for awhile so the temp inside the case can slowly re-adjust to the new temp.
*Above I typed
never... That's because of two problems I did have.
Once, my brother called me from Mexico and said he found a small shop selling high-quality hand-made classical guitars for only a few hundred dollars, did I want one. (I only had a fairly inexpensive one so I said get one for me.) When he got home I picked it up at his house and put it in the back-seat of my car. On the way home I stopped at a friend's house near mine for a minute that turned out to be a couple of hours, leaving the guitar in my back-seat on a very hot day. I went home and brought the guitar in and almost immediately opened the (not very expensive) case and began playing it. Within a dozen notes the guitar began to disentigrate in my hands, the sides splintering, the top shifting out of alignment with the bottom, etc, etc. Totally destroyed (although it still was playable and actually sounded nice, if somewhat muted

). This happened because I did a couple of dumb things I shouldn't have done when I knew better.
The second incident happened when I was on a road-trip one summer to southern Mississippi. I had six guitars on stands in my cheap motel-room (no five-star places anywhere near this little town

). It was sweltering outside with tremendous humidity and of course I had the air-conditioning on. When I went out that morning I took only one guitar with me to use playing with an old blues-guy I knew. When I came back late that afternoon, the maid had come in to make my bed and when she left she had turned off the air-conditioner unit (because I'm sure she'd been told to do so when the air was on in an unoccupied room to save money). The room was stifling when I came in. A 1930 Stromberg-Voisenet in a stand had a top separation and a 1949 Gibson J-45 had the neck-joint begin to give way (having now an action of about 3/8" or more (10-15 mm). Again, human error. (I didn't go in and rant and rave at the desk. It probably would have done little good, maybe get the maid in trouble for just doing what she was told and who was ignorant of what it might do to guitars. I just went in and asked politely that if I'm out please don't turn off the air as I have some fairly delicate instruments in my room that were subject to heat and humidity.) (And then told the maid the same thing.) (And then put a sticky-note next to the dial on the air-unit that said "Please Don't Turn Off.")
Anyway, I wouldn't worry about travelling with a guitar if you take reasonable precautions. And in the end, I tend to think a guitar was made for the owner to enjoy playing it. Not having one with you on a vacation sort of defeats the purpose of its existence.