Best thing we did in the early days was to camp out at the house (and no cheating) and you will find out real quick what you need...
We also had a close by STATE PARK that we called our home base and this is close enough to home to come back for things that you didn't remember to take with you. Also camping close to a LOWES or WALMART will provide for everything you will need when camping...
You will eventually build-up all the necessary things to bring along on the trips.
We use the ACTION PACKER tubs from LOWES to store all of our bring along things that needs to go with the trailers. This way we can just load up the tubs and hit the road...

It will become all second nature after a couple of trips... Making short close by trips are best to do to get to this point...
The main thing is to Have Fun...
Roy Ken