We have two labs, they are in crates when we are away either at home or in the MH. We tried no crates, but they're young and get silly and destructive when we aren't around. After the second call to the veterinary poison control line for ingestion of inappropriate stuff (trail mix/power bars and then deoderant) we decided that crates are much safer. We leave them for a few hours at a time in the MH with the a/c or fan on as long as they are good and tired and not going to be barking or whining or anything. A morning walk before us leaving is a good thing :)
As previously mentioned, if we're somewhere without hookups we do shorter periods of time if it is warmish out. Our Android phones (Samsung Galaxy S4) have temperature sensors and we can leave one behind and set it to send a text if the temperature goes above or below our set threshholds. Nice for if we want to go swim at a state park lake or something close within the CG. Hikes or anything else they come with us.