Tried going for a hike this morning, its roughly 8o% humidity outside, the air temp is 70 dg or so.  I had my camera bag in the garage, which gets some residual A/C as it passes through the ductwork, not a lot, but it's still cooler and drier in there than it is outside.  Before I left I threw my camera bag into the car, turned on the A/C there as well.  Got to my destination, took the D810 out, and blam..condensation.   Pretty irritating as I wanted to get some photos of a waterfall that was at its peak after a recent rain.  I didn't have time to spend waiting for the condensation to clear, so left without getting any photos.

The cars A/C had minimal impact, so it's the fact it was in the cool dry air of my garage that caused this.

Is there any issue with just leaving my gear in a car on a hot summer day as long as it's in the case in order to avoid this?

Not worry about theft...only asking if there will be negative consequences from the leaving my gear in a car on days where its 80-90 dg F to avoid condensation issues.

it took a good 20  minutes or so for my camera lens to clear...

another issue also is if it's inside the house in the A/C, and I see something outside I want to snap a photo of, I can't because of condensation...I've lost some cool shots that way.

I wish i lived in a place with low humidity.

Yup.  Zack pretty much nailed it.  Kinda the same as going out into arctic weather up here.  Keeping your gear at ambient temp saves a lot of issues.  Could always toss your camera into a gallon ziplock with a moisture absorber or two for ~10 minutes to get to the right temp before opening and using it.

That said I don't really have to deal with humidity much unless it's super cold and will flash freeze to stuff.

When traveling on hot/humid days, I will put my gear in plastic bags and let them acclimate.  I hate when my stuff gets condensation all up in it.

All you had to do was wait for the outer, optical surfaces of your camera to reach 64.5ºF.

The situation described resolves itself in a couple of minutes.

A bigger problem is dealing with a camera that's sitting in an outdoor environment that reaches the dewpoint, especially so, if the lens is facing the sky for long stints.

To combat this, wrap chemical hand warmers around the outside of the lens/lens hood, otherwise the photos will start to blur out during the shoot as condensation accumulates.  Sometimes it's a neat effect, sometimes not.  Use the handwarmers to get around it.

It took a good 20-30 minutes for the camera to reach a temp where there wasn't any condensation.  Thats longer than I had to wait that day.

As for keeping it in a place that is at ambient temps, I have central A/C so that isn't really possible, even the garage is cooled off, not as much as the rest of the house but enough because the ductwork runs through it.  

I guess I just need to plan better, but a lot of my trips are spur of the moment trips so sometimes that just doesn't happen.