Help! call from tire place

What was the total series? I don't know where you would find the original tire size but I doubt very much if it was a 12. Maybe a Compact wheel is a 12...maybe.

www.goodyearautoservice.com/content/content.jsp?pageName=TireSize

As you can see, the last number refers to the wheel size. It's not a mix and match number. You shouldn't be trying to stretch a 12 for a 13" rim.

Also, I think it's best to get trailer tires not car tires. It sounds like you have two different size rims? If yes, you might want to buy a used rim so both tires match. Then there is the issue of hubcaps. If you want them to match, the rims have to fit the same cap. Yet another thing to consider.

You can always use the odd size rim as a spare if the bolts match up.

Both my 59 & 64 Shasta had 15" wheels.

You need at least 13 in wheels and the tires you buy should be at least rated for twice the total trailer weight. In other words if the trailer weighs 1500 lbs. your tires should be rated for a bare minimum of 3000 lbs. per pair and more would be better. You also have to have clearance on all sides of the tire so that it doesn't rub on anything such as the wheel wells, springs or spring mounting hardware. Remember that trailer wheels move in all directions as you hit bumps or go in and out of driveways, ect. You also want trailer tires not car tires.
I personally use the highest load rated tire that can be put on my wheel in the size that will clear everything. I don't like to change flats along the highway.