87 Honey 421S

You should have two places to install batteries, one is your chassis or starting battery, and the other location would be your house batteries.  I went online to find some photos and I can only presume that your Honey is a Class A considering that it is GM powered, although I saw some Class Cs, but they were all Ford.  In 1987 Ford wasn't dominant in the stripped chassis market, that was dominated by GM and Dodge.  That was flipped by the mid to late 90s and Ford was becoming dominant in stripped chassis, Dodge was basically not participating anymore and GM was looking to exit stage left.

That is beside the point.  In a lot of Class As, you will have a set of terminals for a single battery.  That should be near the engine, probably under the "hood", and that is your starting battery.  Any 12v vehicle starting battery will do, but because it is a big block engine you'll probably want something in Group 65.  If you want to have reliable starting power you may want to drop the cash on an AGM battery and not get one of those lead acid batteries that you have to top up with water, but AGM batteries are pretty expensive, but they perform well and are robust against extreme heat and cold.

Your house batteries could be in a number of places.  They could be in a box in your entry step.  They could be in a compartment along the side of the vehicle, or they could be in a space that is also under the hood, but located more on the centerline of the vehicle.  It is also possible that your vehicle is equipped with a multi-battery house system.  A lot of Class As were equipped with two 6-v batteries that are wired in series to create a 12v circuit.  Or it could have come from the factory with 2 12v batteries in parallel.  The 6v battery in series is easier to wire up than 12v batteries in parallel.  This was done to allow for longer running time and more battery capacity.  Plus large 6v batteries tend to last longer, discharge a bit more deeply and charge more evenly than a single 12v battery application.

It is also possible that your starter battery and your house batteries are in a commingled spot.  I've seen a number of older Winnebagos where the batteries are under an entry step and both the starter and house battery are in the same box.  The challenge becomes knowing which set of terminals are feeding the house and which are feeding the engine.  Easiest way to determine this is to work one battery system at a time.  Start with your starter battery.  Attach it to a pair of positive and negative terminals and see if you can start the vehicle.  If yes, you found the position and terminals for the starter (chassis) battery.  If not, change terminals and try again.  Once you get your starter battery in place, then install your house battery or batteries and see if your 12v interior lights start to work, if 12v appliances, work, you are in business.

Starter and house batteries are not interchangeable.  House batteries need to be deep cycle, and starting batteries are all about bursts of a lot of power.  I recommend AGM batteries for your house, too.  Same reasons, they require little maintenance, perform better in extreme temperatures.