To give you an idea i have a dandy 6 with tyres on that are a year old,i let the tyre company sell me what they said was safe,with a maximum load of 750kg the tyre people recommended 8ply with a load rating for each tyre of 480kg giving me a total load of 960kg well over the 750kg i needed they cost just over £40 each.
All the information you need is on the side of the tyre,date of manufacture,ply rating and load rating,the problem can be very bad when buying a second-hand unit the tyres could look ok but could be 10 years old.
I have included the next bit as a general info piece.
WEAR & REPLACEMENT
It's a rare trailer tire that wears out, tread-wise. Lack of cornering loads and low annual mileage mean trailer tires generally wear out first from the effects of sunlight and ozone and other environmental factors. The tire industry advises five years as a tire's lifespan due to rubber oxidation. Given low annual mileage, often around 2,000 miles, that means a typical trailer tire accumulates only 10,000 miles in five years when its time is up, even though a considerable amount of tread rubber remains.
Tire companies say time-related damage is difficult, if not impossible, to see. Constant exposure to the pressurized oxygen inside the tire causes rubber breakdown relatively deep in the tire carcass. Outside, ozone accelerates the oxidation process, which is definitely more severe in smoggy urban areas where ozone is prevalent. A stored tire deteriorates faster from interior breakdown than a tire in use, because flexing the sidewall tends to release beneficial lubricants from within the rubber A stationary tire thus has a greater tendency to dry on the surface.
INSPECTION AND MAINTENANCE
Not all tire problems are immediately visible, which is why it pays to periodically inspect tires. A good time for this is while checking inflation pressure. Run a hand across the tread to check for excessive feathering. That is where each tread block has a distinct raised edge caused by uneven wear. More typical of tires on the tow vehicle, feathering is a sign of an out-of-alignment axle or possibly loose spindle nuts.
Check for cracks that could mean your tires are in need of replacement. For example, Michelin's Recreational Vehicle Tire Guide states that most often, the cracks are 360 degrees around the tire, and are acceptable if less than 1/32 inch deep. Between 1/32 inch and 2/32 inch, the tire is suspect and should be examined. If the cracks are more than 2/32 inch deep, the tire should be replaced immediately.
Look also for sidewall and tread bulges indicative of carcass (cord) failure, and don't forget to verify that the valve stem and cap are in good shape. An old, cracked valve stem can break off, leading to a sudden loss of pressure and a real handful of trouble for the driver.
Keep your tires clean by washing them with a soft scrub brush, mild soap and water. Use caution when selecting tire-- care products, and do not use any that contain alcohol or petroleum distillates, which can actually accelerate breakdown of the tire compound.