I use various size plastic boxes from The Range, B and Q. They can be bought for the size of contents, not as heavy to lift in and out, being clear can see what's inside etc. The only thing cannot fit under is the blow up drive away awning, tables and chairs. So at the moment travel on top. Have a bike rack that will take the chairs and tables, bulky but not as heavy as bikes. Have only done this once, and missed the rear view mirror. I could go rear camera, but not struck, i.e. fitting purchase cost, mounting. Don't believe as good as a mirror. So a compromise I suppose.
Slide out look swanky, but you lose as much as a couple of tables of storage etc. Personal choice!