If you search for Rock n Roll beds I've seen a couple of things (YouTube?) where there's a little film clip showing how they open and close between bed and bench seat - very quick and convenient.
I don't have one in my van - the guy that built it made up a seat that converts in his own way. It works well, but takes a few minutes to set up, while a RnR bed opens from bench to bed and back literally in seconds.
Of course the big advantage of ANY bed arrangement that can be packed down to a seat over a fixed bed is that you get extra space to use as living space during the day. When My bed is in bench seat mode than it opens up the access to get in and out, and to get at the grill/hob/fridge, sink without clambering about. You don't get the storage space underneath though of course that you can with a fixed bed.
The ones I looked at with a fixed had the bed up by the back doors, and raised up so that the under-bed storage was bigger and accessed via the outside doors (and/ or some of this space was given over to water tank storage, gas bottle, electrics and so on), leaving the fronto of the van and the side door access to give your 'living space'.
There's plenty of guys on here who've built some great stuff so sure you'll get better answers than mine. I think you have to decide what layout works best for you, and go with that?