Jay, I'm not asking if you think these features are needed. For me, I will not buy a camper without hw heater and 2-way fridge. So I'm wondering what years these items became standard.
Your call but I'd personally want to avoid a built in hot water heater and instead opt for one of the accessory type ones people are using. That way you aren't giving up the space on trips you don't need it. Also its not built into the cabinet making it hard to access/maintain/winterize. That said I don't know when they started, if I had to guess mid 2000s?
I think the furnace option has been around for quite some time so in the eras you're looking its likely an option (that doesn't mean people opted to get it though).
Having an onboard battery would have always been an option, what as changed is how things are tied it & if there is a 110V converter on board as well. Older rigs (like mine) use a simple 12V fuse block and don't have a built in converter (not an issue for me).
Bigger fish to fry in my mind:
Assuming you are looking at a fullsize rig (I thought I saw you looking at a keystone elsewhere) the tailgate opening went from 64"->60"->58". My dodge can fit the 60" fine but some trucks need to newer 58" campers which started around 2001.
Keep an eye on the sideliner in earlier campers, I forget when but earlier campers have some insulation in there that molded out supposedly.
I don't remember the exact dates but in the 98-01' era +/- a year or two (I think it was done by 02-03 though as my camper had its liner replaced around then and doesn't have this issue) they used adhesive backing on the velco for the windows in the sideliner. Apparently this oozes from the stitching and folks find it to be a PITA. Also earlier campers had a fabric headliner that deteriorated over time verse a perforated vinyl that was used later.
Just some things off the top of my head if I was looking to take into consideration.
