Let’s look at this logically. Can the Passenger tires you’ve described be used on a trailer the size of yours? Yes, but, it has to initially be the trailer manufacturer’s choice..
Because>.
Were the ST tires the original equipment tires? If so, Michelin will not warranty any of their tires used as replacements for ST tires that were/are OE tire fitments.
So, will the trailer manufacturer recommend the use of the much larger tires?
The ST205/75D15C tires are bias ply tires - that D in the size determines that. They are rated at 1820# at 50 psi.
The P235/75R15XL tires are radial designed. Because they are passenger tires they
must be de-rated from 2183# at 50 psi to 1985# at 50 psi for service on trailer axles. That is done by dividing 2183 by 1.1. Note; Only LT and ST tires are rated by “load range” lettering. Passenger tires are rated by the load index portion of the tires service description (108T XL).
For helpful hints and ideas about using those tires on your trailer you can read a lot of posts about it on the Airstream RV trailer forum. Look in the tire section. Some of the AS trailers come from the factory with those tires (16”) on them.
http://www.airforums.com/forums/f40/
Airdale
On edit: Your trailer owner's manual should list the minimum clearance needed from the top of the tire to the floor board, normally around 3".
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