If you are looking to replace the heater core due to leaking be sure to take a close look at it. I replaced the heater core on my 2002 Horizon and after words I found that the original heater core in/out stubs screw into the heater core and may have been the source of my leak. You may want to see if your have a hex fitting where the stubs go into the core. If so remove them, clean the threads on all parts, apply teflon tape and reassemble. Maybe you will get lucky!
As for replacement of the heater core about half of the job is done from inside the coach. Disassemble the passenger side of the dash to gain access to the ducting and blower. Remove the blower and you will have clear access to the HVAC plenum. The plenum is held in place by several fasteners (nuts I think) that allow it to be dropped away from the firewall. With the plenum loose it is pretty easy to seperate the plenum halves and remove the heater core. This will be done outside from the front of the coach. Be careful not to disturb any of the blend doors within the plenum. I got a new heater core from Mobility RV. It came with a set of clips that are used to hold he plenum halves together - this is good because they have a tendency to fly off and disappear when removed.
This is not a difficult job and can be done in 1 day using common hand tools. Big key is that there is no need to remove the front cap as proposed by some folks.
Let me know if you have questions,
2011 Itasca Meridian 34Y
2006 Jeep Liberty