Adding a water heater isn't something a lot of people do 'round here, so we don't have much experience with it. I'll point out what I can about our stock-trailer installation:
First, our water heater is installed directly over the wheel well and axle
, which makes sense because that would also center the 50 pounds of water weight
(8-1/3 pounds per gallon) over the axle
, which helps make it easier to balance the trailer for safe towing.
The water heater is directly below the sink, so we don't waste a lot of hot water or gray tank capacity waiting for the water to "get hot."
Our water heater sits on a platform made from OSB that supports the weight
of the heater and creates a space under which the water pipes and electrical
I added a Hott Rod 110v electric element (Another brand name is Lightning Rod) to our water heater to cut down on propane
use and simplify trailer setup. All I have to do is plug the trailer in and it takes 3 hours to get the water from cold to hot, no lighting
Be careful to cut a cardboard template for your fiberglass cutout and test-fit it to your heater before making any holes in your hull, then trace the outline to your trailer with a grease pencil and drill pilot holes at the corners and check them from the inside to make sure your small holes are coming up in the right place to fit your heater. Small holes are a lot easier to patch than big ones are.
Be very, very careful about running your gas piping. It sucks to fill your trailer up with propane
then watch it blow up. Particularly if you (or a family member) are inside at the time. This is the one task in trailer maintainance where a small, less-than-obvious mistake or omission can kill. If you are not an accomplished handyman, if you are not familiar with the peculiarities of gas plumbing, reconsider doing the job yourself and hand it over to someone else.