I discovered a couple of weeks ago that the dash heater core on our 1999 U270 was leaking - saw slow drips out of the dash AC drain hose on the passenger side behind the upper corner of the generator door. I ordered a new one from FT, and got it replaced today.
It was fairly straightforward to replace it, but there is a lot of stuff to remove to get to it. The good thing is that you can do it all inside the coach, standing or sitting, in air conditioned comfort (it was really hot outside today!) Here's a blow-by-blow, if you need to do it:
Outside, through the generator door (passenger side), the heater valve is easily accessible. I clamped the heater hoses to keep the engine from draining, and removed the heater valve to allow the core to drain.
Inside, detached the two swing-up dash covers from their hinges, then the passenger side vertical trim panel nearest the windshield, and the horizontal trim piece over the breaker panel cover, removed the defroster ducts, and then was able to pull up the padded dash cover which is held down by velcro.
Next, lifted out the center electrical service panel and removed the center console, which is held onto the floor and frame by 6 or 7 long screws.
Labeled and removed all the air duct hoses from the air box.
The air box is held in place by two screws through a vertical angle into the dash frame on the left side, and by two screws into the floor at the front. Once it's loose, it can be eased forward a little which makes access easier, but it was not possible to move it very much, since the stiff AC hoses prevent any significant movement.
After loosening the box, it's simply a matter of removing many many screws holding the sheet metal pieces on and finally getting access to the top of the heater core which you can see through the flap opening after taking the first sheet metal cover off.
You have to take the control levers off the flap motors, and lift out the two flap assemblies, so it's helpful to mark the flap positions before removing. (I forgot to do that, but it's not hard to figure out the right motor / flap positions by moving the controls to different settings and watching the motors.)
Finally, to remove the core, I took all the screws off the right side plate, and then detached the hoses (be prepared for more coolant draining!) and worked the core up from the left side.
New core went in the same way.
Right hand side of the generator opening