The 1990 era 460 does not have automatic belt tension pulleys. It should have two belts and the adjustment is on the alternator and AC compressor. At least it was on my last RV on the Ford F53 chassis. It might be different on the Oshkosh chassis but I had an old thunderbird with the 460 and it was also the alternator and AC. On mine I changed them from above with the exception that I crawled underneath to verify that the belts were in the correct groves on the harmonic balancer before giving the final tightening and starting the engine.
Mike