All RV a/c units should work fine. You may need to reinforce the skin--I've seen some pretty bad original installations that don't have suitable bracing around the 14" square vent hole. You might also need to trim the a/c hole to be a square 14".
You'll have two real issues and one installation consideration:
1. Drain--some modern RV a/c units don't use a condensate drain, they evaporate it or allow the compressor fan to splash it away. Some vintage refurbishers have complained that this often doesn't work perfectly and condensate running down the side of the shell will leave a stain. If your new a/c uses the drain line, you'll have to mechanically get the original one to extend to the new port.
2. Mounting plate--the original a/c installation used a large sheet with ribs to support the unit. The ribs were for attaching to cover and are part of the sheet (can't be drilled and removed). You can leave the sheet attached, but you'll have to deal with the appearance of the ribs. I gently knocked mine down flat and although it doesn't look good from the top, you can't tell from the ground. If you remove the sheet, that's when you've got to install skin support inside between the inner and outer shell. You'll also have to somehow fill the rivet holes left from removing the sheet, as they are 6-10" outside the outline of the new a/c.
3. Many of the new a/c models have integrated the thermostat, so you won't need the old wires or thermostat. You may be able to wire the old control into the new unit, if you prefer the temperature to be controlled on the wall versus the incoming air at the ceiling.