Why not call the shop where the 5'ver is and ask them to look at the tires and tell you what the size is?
Sounds to me like the easiest...and best option!
However, if they are Castle Rock china bombs...they are most likely 225/75/15's!
I put 'E' range Hercules SP2's in that same size on my 5'ver...love them so far!
It had 'D' range tires on it before but wanted the higher PSI of the 'E' tires!
And they were only $1.00 more each than the same tire in 'D' range.