Replacing heater with a/c?

Since you don't move it just install a mini split heat pump. Then you have heat and AC. Most of them have revolving louvers so the air distribution in a small camper world be very good. Here is one that runs on 120V for less than $1K
Mini split opps, this one doesn't do heat.

This one does Heat pump

Fix the heater or just get a fan.

Just replace roof top A/C Unit.
It would have to be a non-ducted type because you already have a non-ducted.

If this mountain parked RV is 30A that is best you can do. IF it is a 50A then you could add another non-ducted roof A/C Unit. They fit into same opening as a roof vent uses (14" X 14")

Also look into a split ductless system. Samsung makes them amongst others. You have your condenser sitting outside and wall mounted evaporaters on the walls inside your unit. Put one in front and one in back.

I cannot believe the comfort that a sitting-on-the-floor 5000 BTU air makes my bedroom. It is flush to the outside and indide it is covered to make it an bedside night stand. Outside is covered with a perforated stainless-steel louvered plate. Why should the pork & beans in the kitchen be chilled? A/C works best for me from the bottom up. Why should the ceiling be chilled? I am cooling my rig far better on half the power.

Basically you want to add central air to your trailer. Doable? yes. Cheap or easy? no.

I understand your thinking on the portable, they are inexpensive and easy if you could just stuff it in there and hook the ducts to it. First thing is the portable will have to be as big or bigger than the roof air. Second the fan in the portable isn't made to push air through ducts, so a different blower will be needed. Third, our 5000 BTU unit wouldn't fit where our furnace goes. Even if it did the back end has to have fresh air flow for it to work.

Call the HVAC company and tell them you want to put central air on the trailer and they may have more ideas but just like a house you would have a condenser unit outside and the evaporator in the plenum.

With a portable you would take the cover off, have the refrigerant pumped out. Cut the lines to the evaporator and remove it. You would also need to remove the controls to be placed inside. The condenser unit will stay outside like on a house and tubing added to reach the evaporator inside the plenum. You will likely have to custom build the plenum unless you can use something from the heat system. The controls will have to be mounted inside with their wiring extended back to the condenser so that you can operate it from inside. Once it is done have the system evacuated and charged up, then you have a central air system like a house has except the condenser unit isn't as pretty. Not too bad if you have the tools and skill needed.

My first thought would be to add a inexpensive window unit on the ends of the trailer to help out the main A/C. Those may not be allowed so you could get two of the portable units with the hoses that go to the windows. Either would be easier and less expensive than adding central air or gutting a window unit.

There are a few small free standing AC units that run cool air through a plenum from outside to inside. I suppose you could adapt one of those to feed through your heating ducts. But IMHO way more work than its worth.

I have seen house boats with a furnace on the dock ducted to the boat. don't know why an AC couldn't be done the same way.
bumpy

I would not use the existing ductwork, because it is not insulated, and it will generate condensation on its outside.

There is no inexpensive EASY way to install/add a AC to connect to the Furnace floor ducts. RV's are not designed to have that done. There is no room AT the existing furnace area and that area is where you would have to have the forced air start at. Your best bet, is to either replace your roof AC with a larger unit or ADD an extra Roof AC and have 2 if you need that much cooling. The reason local HOME AC companies cannot do that job is because there is NO AC unit designed to do that. They are also not qualified and understand how RV's are put together to do what you want done. Doug