I just pulled a HWH out of my "new to me" '75 Trade Wind the other day. I had the same issue of not being able to get it to budge, so I gave it a little push from the inside and it poped loose. Mine was sealed to the outside wall pretty good with weatherstripping and caulk. My HWH is located right under the sink so I was able to easily access the back of it from inside.
If you haven't ordered a new one yet, check out AdventureRV
they have some really great prices and if you order online it is really quick. I ordered a replacement (doesn't include a new door, didn't care as I will use my current stainless one) 6gal Atwood with elecronic ignition for $379, plus ~$30 shipping, and no tax (if not in Tennessee). My local RV dealer that I normally purchase items from was $550 not counting 6% tax (their's included a door, again didn't care). I ordered it on Monday and it was at my door yesterday (Thursday.)
I also just purchased a new Intellipower 9200 converter from my local dealer last week for $250. When I ordered my HWH on Monday, Adventure RV had the same one listed for $150!