Gary, there's a difficulty cornering with these 'trailing racks' on HUGE American motorhomes because of the enormous rear overhang - that isn't the case with most normal UK motorhomes, which, as you say, are the size of 'normal' US estates!!! There's no problem using these on an ordinary UK motorhome. Mine's an AutoSleeper Rienza.
Stability at speed isn't an issue - the 'bike becomes a single-wheel trailer with no rearward weight to make it unstable. There are legal speed-restrictions when towing anyway.
There will be minimal tyre-wear but the rear wheel is only rolling - not braking or accelerating.
All motorhomes have an on-road weight limit, ie weight of vehicle + weight of conversion + weight of accessories (water, gas, etc) + weight of passengers and their personal luggage. It's this last category which is VERY slim on some conversions, very few kilograms in some cases. I'm managing my outfit by reducing the amount of water I carry. Search this site for more advice on "Weights" - there have been a great many, very enlightening, posts on overall weight limits. The penalties for exceeding maximum vehicle weight (if you're unlucky enough to get stopped by the "weight police"!) are swingeing.
One final consideration if contemplating 'trailing' is the final drive of your 'bike. Some machines use engine oil-pressure to lubricate the gearbox, in which case the chain should be removed (because the engine won't be running and the gearbox bearings could be running 'dry'). Fortunately for me (and you'll forgive a moment of smugness...) my 'bike's a BMW with shaft drive and separate gearbox lubrication.
Good luck with whatever you decide - it's good to see a few more 'big-bikers' motorhoming!