Hi Matt, and welcome.
A couple of questions, if I might.
Firstly, how many billy lids do you have, and what are their ages?
This will no doubt be relevant to your choice.
Some campers have capacity for zip on rooms, which an accommodate bunk beds, for example.
Others - e.g. Track Shack,have multiple elevated sleeping areas.
I would suggest as follows:
1/. Work out your design criteria; 2/. Talk to manufacturers you are interested in regarding price / features / cost of 'extras'; 3/. Compare with what you can get within your price range on the second hand market, with similar cost / features, even if a different make.
Then consider how essential buying new is to you. For some it is an absolute, for others value for $ used makes that direction a 'no brainier'.
Either decision can be valid, but I recon it is important to consider both options.