Although I understand your argument that replacements on the road may be difficult, what are the odds that both units go out at the same time? Your unit has ducted air right? I have the same year National RV as you and both my units still work fine. I wouldn't even consider just replacing them just for the sake of age. When they go out I'll replace them. It's not like only running on one AC unit creates an emergency or leaves you stranded.
Sorry, I realize that doesn't answer your question, but I was just having trouble understanding the logic.