In your situation (trailer permanently parked) a 12 gallon electric residential WH will work just fine.
Water connections are on left side......
Cold inlet bottom connection
Hot outlet top connection
Suggest you install shutoff valves so Wh can be isolated in future if needed.
Drain should be piped outside
The Temp/Press Relief valve on top right.......
That needs to be plumbed with piping to exterior and down below side of trailer.
Then you need to AC wire from circuit breaker to ON/OFF switch to WH t-stats and heater element.
Outside access will no longer be needed so that 'opening' should be closed up to keep weather/critters out and insulated.
Good luck on the project. Lowes has PEX piping and fittings
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31