Perfectly fine to store that way, many of us do and their website says it's okay.
Close How much storage space do I need to store your dolly?
Our tow dolly is the only tow dolly on the market that can be stored in a vertical position requiring a 25 inch x 102 inch footprint. Our dolly will easily tuck under the rear of most coaches about 85%. Our tow dolly is 102 inches wide, 92 inches long and 27 inches tall to the top of the fenders, weighs 330-380 pounds. It will stand on its on without an aid but we recommend restraining it in some manner.
1996 Newmar Mountain Aire. 3760 CA
2006 to 2015 Boxer named Cash (aka Cassius Clay)
2016 - Boxer named Chevy