The tow hitch was off a Patrol and longer than the VW one. The VW will accept 350kg ball weight and the wheel plus Jayco weighs less than that. There was a little free play, which couldn't be avoided to allow the hitch to swing. I think there is only about 1-2cm sideways movement at the top of the wheel. Dad (the inventor) used to make mobile crane jibs from scratch, so I trust his expertise when it comes to how strong to make it.
Please take this the way it is intended. There is a set of rules for lifting and there is another for carrying. Mobile cranes are designed for loading/unloading trucks etc. so I don't doubt Dads spare holder would be strong enough to support the spare for years. Once you put carrying into the mix , the rules change. You have a wheel about 15-20kg hanging off a piece of steel about 300mm long , so that's where the flex comes into it. I might be wrong but I don't think a mobile crane can travel with a load, so flex doesn't come into it.Before you ask, yes,I have worked with steel. I don't care if I'm right or wrong , just make sure your butt is covered insurance wise.