First you'll need a part number from Monaco, contact them and ask for a quote, it will be high but it will have info on it. I got a quote +$6K
You may get lucky and find a used one, I had a radiator failure in May, I contacted Visione Auto Mart in KY and at first they thought they had one to fit but turned out they didn't. The quote was higher then what I eventually found new.
Source Engineering probably will be able to get you one, they quoted me ~$3400 plus shipping
I ended up buying new from CG&J in Alabama, initially I was going to have mine rebuilt, quote was +$3K but in the end there was a Monaco Windsor like mine that had just had one done and they could make me a new one for a lower cost $2500 including shipping. Fairly fast turn around.
If you have a local radiator shop they can probably rebuild for you cheaper.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi