Hi Rab,
I've been along that road a few times without the van and always said to the missus, " Can you imagine bringing the van along here ! ".
I'd love to be able to say to you that it's no problem, but I can't, sorry. The gradients aren't too bad, it's more the narrowness of the road that worried me and the tight turns.
It's definitely doable, no doubt about that. But not sure it's worth it, for me.
Have you considered Faskally, which is just off the A9 and a doddle to get to ? I've never been, but reckon that it has most, if not all of the facilities that Tummel Valley has eg, pool, bar, games room, playareas etc.
Good luck with whatever you decided to do.
Cheers,
Alex.