Your description of the trailer needs a bit more definition. Is it a rear bath or a side bath model? this makes a diffrence on the water heater ease/difficulty of R&R. Also if you update your profile so we know where you are members may be able to reccomend an alternative facility.
As to the furnace, it should not take them 2.5 hours to remove. If they are charging $75.00 per hour the $560.00 per appliance is 7.75 hours?????? Each??? I would be very leery about having them work on my trailer.
The furnace is usally under the stove/sink in the kitchen. Due to the fact that they need to be worked on from time to time Airstream usally did not make them too hard to remove. On my last 2 trailers and on the MH I currently have the furnace is behind a panel held on with 2 screws. There are 2 screws that hold the furnace to the floor, and the vent screws that hold it from the outside. There are 3 ducts and 1 wire harness.
I had already removed mine as it was not working, but I think it only took 1.5 hours to install and test a new one. And I do not do this for a living.
The water heater is whole diffrent can of beans. Depending on location and the heater intalled at the time it was built, or if the prior owner had it replaced, the newer heaters may not fill the hole. I am facing the conundrum of how to address the situation myself. I have seen where the new heater has been installed in the old chassis so the exterior is the same as before. I have no idea how many hours this took. Before they go pulling out the old one you may want to ask if the new one will be an exact fit.
Another option is to get a second opinion. You may get the same costs and labor, but then you will know that the original shop is being fair.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato