When I started my build, I had a short list of "must haves" 1- bathroom with flushing toilet & shower, 2- big fridge, and 3- large fuel capacity
Spent months visualizing & planning before I tore anything out. Saved & reused as much of the original cabinets as possible (you can't beat the quality)
1- a 1 piece fiberglass unit out of a water damaged truck camper came my way for $100
2- 8 cf RV fridge just fits above the wheelwell, I also use a Norcold 12v fridge/freezer in the passthrough area on longer outings(great for bringing Halibut & Salmon from Alaska!)
3- Adding a van style tank is pretty straightforward(gets you about 30+ gal) & since I disabled the rear doors( bathroom partially blocked one) I had a tank made & vented it into the generator compartment( 55 gal & you'd never guess or smell it's there)
Also, as far as AC units, check out Dometic Cool Cat... 10K BTU AC & heatpump, I installed mine in the top of the tall cabinet behind the driver, a tight fit but, I absolutely love it! Nothing beats a heatpump on chilly (not freezing) days, providing of course you have shore power.
So, get your list together & have fun! Look forward to seeing your progress...