Hi
There are lots of choices about what to do / what to use / how do do it. Without standing in your trailer and looking at all the things you want to do throughout the trailer, there is no way to come up with "one answer".
Best guess:
Battery monitor: Victron BMV712
https://shop.pkys.com/victron-energy...y-monitor.html
Disconnect switch: Blue Sea
https://shop.pkys.com/Blue-Sea-6006-...ch_p_1622.html
12V fuse panel: Blue Sea
https://shop.pkys.com/Blue-Sea-5026-...ts_p_1615.html
120V stuff likely comes from Home Depot.
SeaLevel stuff is commonly used for tank monitoring these days. You have a stick on that goes on the wall of the plastic tank. It has to match up with the tank dimensions.
Battery wise, I would suggest you consider lithiums on this involved a rebuild. That's not to say they are a must, just something to understand and to consider.
Bob