There are standard interoperability channels on conventional UHF, VHF, and 800mhz systems. Most grant funded communications initiatives require these channels to be programmed in exchange for funding. 800 systems almost always need to be P25 compatible for funding. What is it that you seek for "national standards?"
I'm not sure that needs to exist either. I have three radios in my ambulance because that's what we need. VHF makes sense for us given the terrain, but the urban area to our east is on 800s. We make it work, but it doesn't make sense for us to all be on the same system, each system has different needs.