So just for a change I'll disagree with Mike
Historically trailer and caravan tyres were given a exemption or boon on weight loadings. Because they aren't linked to a transmission or steering they were given an allowance above the quoted weight on the tyres. That time has passed and no such allowance is given now. I'm not sure when it lapsed.
Though I would be more than happy to tow with tyres of a suitable rating.
For heavier folding campers or caravans specialist tyres with either 6 or 8 ply sides are used These are often listed as light truck or or or or I can't think of the other term. Is it just me or has Google suddenly become less effective.
A list of tyre ratings here: [You must be registered and logged in to see this link.]
All you have to do is make sure that the load index is sufficient.
I bought a new set of Mini sized radials for my old Green Un tyre size 145/80 10 xx ? As the unit isn't close I cant see the speed rating R,S or T and I can't remember the Load rating it was somewhere between 62,63 or 64. That is between 265Kg and 280Kg so 530Kg and 560Kg for the two tyres.
Unless you are using the unit for some unusual purposes the tyres will be killed by Ultra Violet degradation in the end why over speck?
When you come to buying the Tyres one option that constantly crops up with budget trailer tyres are Bias Belted tyres. These are almost like a crossply tyre in construction but with an additional radial belt. Crossplys and Bias Belted would look more "period" on very early Dandies but while not been able to state definitely from day one Dandy used radials from very early on. Though the crossplys and biased belted often have a higher load rating, if you tow in France where there is a maximum 81mph towing limit or were a bit cursory with your attention to speed limits I think (not sure) you are getting to the end of bias belted and crossplys speed limits.
Below is an advert showing crossplys, bias belted radials and reinforced (don't use the term reinforced that is now been used for the new generation of run flat tyres) radials. [You must be registered and logged in to see this link.]