Atwood is the way to go.
Replace with same size (gallon for gallon) and it'll fit in the same opening.
Make sure you install a "leak-pan" under the new water heater (what was AS thinking?).
Combination gas/electric has the advantage of using shore power instead of propane, when convenient. They use quite a lot of propane. Make sure your electrical system can handle the additional 1500
If you can't do it yourself, have an RV shop install it for you. They'll probably want to sell you one from their stock, though.
The only thing I didn't like about the new Atwood is they don't offer a chrome (or aluminum) exterior cover. White only. There's a marketing potential!