i would guess that your originals were load range C tires in a size 225/75 R 15.
the stated sidewall max pressure is 50 psi for load range C tires .
if you purchase load range D tires they are rated for 65 psi
if you purchase load range E tires they are rated for 80 psi .
http://www.tirerack.com/tires/tires.jsp?tireMake=Power+King&tireModel=Towmax+STR&sidewall=Blackwall&partnum=275R5TMSTRV2&tab=Specs
your best situation would be to replace your tires with a tire that has the highest load rating in pounds that fits your 15 inch wheels.
a very good choice would be of any brand in a size of 225/75R 15 load range E which has a load rating of 2830 pounds at 80 psi .
the brand is not as important as the weight rating in pounds .
you can run those E rated tires at 80 psi for the maximum weight capacity .
there have been ZERO wheel failures running steel RV wheels at 80 psi caused by inflations at 80 psi .
most steel wheels do not have a maximum psi stamped on the wheel .
some steel wheels do have a maximum weight rating that is independent of the psi max ,
some aluminum wheels have a max weight rating and psi stamped on them .
the maxxis lovers will certainly chime in .
there is NOT a single tire test that shows a maxxis tire in the top 50% for quality. NOT ONE .
Those promoting maxxis have their own agenda based on their feelings but no reviews or research substantiates their opinion .
pick any brand .
go with the highest weight rating .
this will give you reserve tire capacity .
