No harm in running them for extended periods, days even. That's what they are designed to do.
Longest I ever ran mine was about 7 hours and that was an accident. We were in the South West somewhere. Was very hot as we were getting ready to leave. I flipped on the gen and the roof airs so the DW would be comfortable as I disconnected everything, moved the coach a little and hooked up the car. Forgot the gen was on and took off. Was amazed how cool the coach stayed with very little help from the dash air. Kept upping the temp setting so it would not be so cool.
Didn't realize it until I started to hook up in the new campground. I was wondering who was running their gen at 6:00 at night in a full hookup park!!
At least I didn't leave it on all night