The main issue is often the weather. If it's hot, there aren't many good options. Leaving your dog in the TD with the air conditioner on is one possibility. We haven't yet gotten a comfort level with that ourselves.
Our tow vehicle is a windowless van, so it's pretty easy to orient it to minimize heat gain on sunny days (and we look for shade, when there is any). So if it's not too sunny or too hot, we'll leave her there at times. On cloudy or rainy days we'll leave her for up to three hours in the van, but we leave a 12v fan running while we're gone, and we have a spill proof water bowl for her. We just don't trust sunny days to leave her for more than 30-45 minutes, though sometimes we'll tag team checking up on her if need be.
Spring and Fall have more days that are conducive to leaving her in the van, so that's the best time to travel with a pet.
If you don't have a windowless tow vehicle, I'd look into various window tinting options. Many legal options will drastically reduce UV and infrared gain, so that can help make things more comfortable for your dog while you are gone.
Part of any answer is a true assessment of how well your dog does with being separated from you. We left Lana in a kennel next to Mammoth Cave, KY while we did a cave tour. It was tough walking away listening to her howl, but she seemed to survive just fine. That was for about an hour or so. Our preference is to leave her in a place that she is familiar with, but that was a sunny day and we weren't gone that long. But was she happy to see us after the tour!