I spoke with a very knowledgable and friendly Fuso Rep today out in New Jersey. He told me that they use a "damage and a progressive damage" when looking at warranty claims. So yes, 19.5 super singles do not effect your warranty claim on the A/C- but could be considered for engine, transmission, wheel bearing, etc. He forwarded me the following and told me it was all covered on the mitfuso.com site. I looked, but could not find it. But what he sent looks like it was written by an attorney.
Warranty Statement for Use of After-market Equipment
Under the terms of MFTA's New Vehicle Limited Warranty, modification of a
chassis or chassis system or usage of any after-market part, kit, or
system is solely the responsibility of the installing dealer, upfitter,
and/or customer. MFTA makes no warranty for these modifications or parts.
Any MFTA component that is normally covered by an MFTA standard or
extended limited warranty which fails or becomes damaged due to a
modification or due to the installation or use of an after-market device
will not be covered. Further, any progressive damage caused by the
modification or by the installation or use of the after-market device will
likewise not be covered by MFTA. Please refer to the MFTA New Vehicle
Limited Warranty for specific coverage details.