The out-come of my problem with the tyres for the Thomson was that in the end I took the 15 inch wheels to an Engineering company who cut the centres out then welded new 14 inch rims to them, when completed I then bought two new 185 x 65 x 14 light truck tyres and had them fitted.
Doing this still didn't solve the problem as I then had to cut 40mm out of the top of the wheel arch on each side of the caravan so the tyres would fit over the brake drums, in the end every-thing came together very well and the smaller tyre diameter made very little difference to the ground clearance of the caravan.
The Thomson travelled very well behind the car at an average speed of around 80-90 KPH , in fact out of all the caravans we have owned over the years ( 14 ) I think the Thomson is the best to tow, it sat behind the car in a straight line and never swayed or bounced once.

The new wheel is on left.

The 40mm cut-out from top of wheel arch.

The new wheel in place

Pictures of the 'van

Wild-flower Country.... Western Australia.

I have changed some of the interior fittings of the caravan to suite our own requirements, the major change was the removal of the centre pull out double bed, I replaced this with a club lounge and a drop-down table, a hand basin with tap from a 12v water pump was also added to the wash room, along with a repaint and new floor tiles. 

We hope these photos are of some interest to you and other Thomson Caravan owners.

Cheers and beers Jim & Jean