if ya need to fill some cracks up in the camper for air leaks. i would suggest some "closed cell" spray foam. not open cell spray foam. you can normally pick up a can at local hardware stores. **generally use "great stuff" in red can.
to many times trailer doors and like have slammed shut on my fingers and others, more so with wind, since this is a DIY camper. i would place an EYE bolt or a hook. just off to the side of the door. that you can use a bungee cord to hold the door open.
making a small step, out of some 2x4's or 2x6 or even 1x6's for getting out of the trailer can make your high step in easier more so if ya end up in mushy ground right in front of the door.
if you have old piece of carpet or an old rug, to toss in front of the trailer door. also helps with mud.
always having a stack of newspaper or old phone books, works nice for starting fires. to a point of having a box or a shelf to put them in helps.
if you want trailer for most part level after parking it. having some jack stands or some 4x4's or like lumber and some thinner pieces to place under trailer to jack things up and get it level. you need to do this, but just saying.
for myself found that just getting cardboard boxes. ((nearly every business toss them out into the garbage)) and just using them for dishes, and taking some dish towels or regular bathing towels and wrapping any glass wear up. or any pans up. and putting them in a box. ya get your towels plus dishes.
i hate blow up mattresses, way to easy to puncture. more so if you get a leak and no were to fix it. if you can find a local bed place that makes there own beds. you might be able to pick up some "foam" a few inches thick. it is not a perfect bed, but gives some comfort. and light enough that you can flip it up and out of the way if need be vs a regular bed mattress that is generally heavy and awkward to move around more so in a tight space.
if you do have cabinets with doors or regular drawers, with either magnetic or little clip to hold the doors / drawers closed. you might want to see about bungee cord being place around everything. to make sure nothing comes open mid trip as you are going down the road. it might not look the greatest. but it sure works a lot better vs picking up a pile of mess on the floor.
a card table. with 4 folding legs say 3 foot by 3 foot. works nice for an inside table. easy put up, light, and easy putting back down and getting out of your way.
to be honest, i also prefer open shelving. no doors / drawers myself. if you made a shelf kinda like inventory shelf you will find in most business made out of say 1" x 1" boards. with a little lip on the front side. and use some bungee cords across the front of it. to hold things in. it makes for getting sleeping bags, blankets, and longer stuff in and out much easier. it may not look the greatest. but for me, it is more about use, than anything.
============
with reg, it sounds like the tongue on trailer is rather light. when packing things up in it. heavy stuff i would place in the front.