Most of the conversions have a full length box along the offside walland also plays host to the chemical toilet...lovely.
This fixed box has a sliding portion which pulls out to make a second seat which in combination with the table top provides support for the second berth.

The "Kitchen" is a cantilevered store fitted to the rear three quarters and allows the feet of the occupants to fit under the worksurface. Both occupants sleep with their heads at the cab end.

The kitchen store has just enough space for a water supply, sink cooker combination and camping-gas cannister plus a few pots and plates. There was usually an elevating roof fitted and most users will need it to do anything like cooking etc at the minimal kitchen space.

Adding a second battery is no big deal and a removable unit would make the most sense otherwise source/fabricate a second underfloor battery mount as used in the original vehicle and fit it with a split charge facility.

Either way you will need to rewire most of the electrical system so that the new battery drives the radio, interior lighting etc and the main battery supplies the vehicle for starting legal lighting etc. Small camper conversions appear simple and cheap and can be done that way.

But the mainline units won't normally leave you with with a flat battery halfway up a scots glen in the pouring rain because you forgot to turn off the reading light before you went to sleep...by all means save money but think about the cause and effect of cost saving.

However, a "picnicar" makes lots more sense as the bed option is not required you can have twin comfortable high back recliners with arms and seat belts so can legally carry four, most small camper conversions are sold as two seat/berth. A simple folding table is easy to arrange and a portable cool-box fridge will keep everything fresh, you can fit an extra battery if you want but it is a luxury you may never need.

Agreed the portable fridge/cool-box can be part of your camper but it then restricts another option or forces an expensive work round to keep the space and have the fridge.