The 3 year old Dometic 2820 Fridge is stinking up the Barth and looks like I need to replace the cooling unit. Very suprised as this unit is fairly new. I am second owner so no warranty for me.

How hard is it to replace the cooling unit. I have found them on Ebay for around $300 to $400 which is cheaper than a new unit.

"How hard is it to replace the cooling unit."
I don't know about yours, but my 10-yr-old Dometic was replaced 2 years ago:
1) After unhooking elec/gas in the rear pull it out of the slot against the wall.
2) Lay it down in the aisle (or remove a window and hand it outside).
3) Unbolt the cooling assembly which is essentially a flat coil with tubing and propane/electric connections.
4) Pull the sensor through the ref. wall.
5) Install the new cooling unit by pushing it into the indentation in the refrigerant foam (styrofoam?) where the old one came out then bolting it in place on the rear of the box; inserting the temp sensor; and sealing the coil to the box so it will actually transfer heat.
6) Put it back in place and hook up electric and propane.
Mine had no chimney as such; the roof vent is above the compartment. It is not as simple as written, of course, but there's nothing hard about it, e.g. no refrigerant to add. IF you have the right replacement coil it's easy - it's just an insulated box with a cooling coil pasted on the back. Be sure to get the stuff that seals the coil to the box - I don't know it's name or where you buy it.
nb: "$300-$400" sounds like a really good deal.
Hey: what's the worst that could happen? It doesn't work now, so you can't hurt it. The worst: calling the RV Repair Guy and admitting you can't do what he has been trained to do?

I replaced my cooling unit last spring with one purchased from this ebay source:
- http://stores.ebay.com/rv-propane-refrigeration -
The operation was a complete success.

I had help to lift the fridge in and out but installed cooling unit myself on the floor in the coach as Gunner describes. Cover everything in work area as you will be working with some sticky sealing stuff. Complete instructions came with the unit and were quite easy to work with. Steve (the seller) had said he was available by phone if any questions came up during the installation. Delivery was as expected but not fast.