Our 20' Argosy MH had a 16000 BTU furnace and a 6 gallon hot water heater. When you buy the hot water heater make sure it has the electric start so you can start it from INSIDE the motorhome! Our first MH had a hot water heater you had to light from outside and that was no fun, especially in the cold
We haven't shopped for an oven/cook top yet (or any other appliances for it, we're waiting to make sure we can get it driveable) but they are all pretty much standard in size for the RV industry.
The trick for your furnace will be to ensure the new one has duct openings in the same location as the original.
The fridge is a different story. Someone replaced ours with one of those small 120vac under counter units that you see in motels, etc. Thats going to get tossed and the area reworked to install a slightly larger fridge. We think the original size that Airstream installed is just to small so I'll rework the counter area to install a larger unit.
Hope this helps.