First I am a master electrician and I used to do a lot of power on sailboats so I am a pretty good idea of what works.
First I would save the money and go lifepo4, a 100 ah battery will have the same capacity as your 200 amp agm, and weight a 100 lbs less.
Next applies to what ever battery you want. install the battery and get a fuse block to run the + wire to, you dont want a melt down. Next run to a blue sea fuse block, they are simple and work very well. Then figure out where you want your switch bank, and run a wire from each fuse to the switches. Typically you will want a water pump switch, a master light switch, and a switch for power ports ( either lighter style of usb ). The run a ground and a wire from each switch to what it feeds. You will need a ground block to tie all the grounds to and then run a larger wire to the battery ground. Dont be afraid to look up marine wire on amazon, the 14 or 16 gauge 2 conductor cable is surprisingly cheap.
If your running and inverter take its positive wire to out side of the main fuse block. Remember inverters draw basically 10 times the amps on the 12 volt side as they do on the 120 volt side. Ie you want to run something that draws 5 amps at 120 it will draw 50 at 12 volts. to figure amps from watts divide the watts by the volts.
Btw a 200 amp agm will give you about 100 amps or so of usable power, that is a lot esp for a small trailer like that. your water pump will draw about 5 amps but only when running, lights if led will only draw 2 amps. your phone might take an amp