Let me guess,
I have a 31' RV with one 15,000 btu AC?
It is 102 outside and my two AC units only cool to 85 or higher?
one unit went out and it is too much too much to replace now, so why I can't I use a portable one?
OP never said what size Portable unit but unless it is gigantic, it may only cool a bathroom if the door is shut. I don't see any portable unit cooling a 31' RV with temperatures over 85 degrees.
Also OP would need a pretty good water drainage system, or plan on dumping lots pans of water every hour or so.