Tare weight/ gross trailer mass

I was hoping someone could let me know what a "rule of thumb" would be to determine the gross trailer mass of a van with a Tare of 710kg if the official gross mass was not known.

I am looking around for a 1980's Viscount Sea Breeze / Regency / Aerolite / Venturer pop-top (12-14ft with sleeping for four) to (perhaps) tow with a 2.0L Ford Focus Sedan 2010 with a towing capacity of 900kg.

The owner's manual states towball weight of 90kg and tow hitch mass limit of 1000kg. In real terms, I think the vehicle will more than adequately tow such a van, but the legal aspect of the weights mentioned above is a concern and I don't want to go out and buy a van before I know for sure. Any advice / comments will be welcome.


Craig Naylor, WA