Hmmm, that's an interesting question. I've never heard of interior design templates.
Buses of various lengths, makes and models all have their unique system placements, tanks, outside storage compartments and body features.
Where you can fit a drain line, a tank, bolts and such depends on what features are on the bus.
You won't know any of this until you own a bus.
It's less of design first, buy and build and more of buy and then design and build.
Even if you buy and then design, at least for me, it's a very dynamic build experience. Lots of "oops", "that won't work", "hmmm, I didn't think about or know that was there" kind of obstacles.
If you want to simply determine if you want a mini, a midi or full length bus, I'd get graph paper or design software and plan it out.
I used my home to determine sizes. In my home kitchen counter is 36" high and the sink bottom is 8" below the counter top. It's uncomfortable because I have to bend slightly to do dishes. So, I made my bus countertop 3" higher and I can touch my sink bottom while standing straight.
Design also depends on your priorities. Some people have kids and need bed and storage, others work remotely and need workspace and some may love to cook and entertain so the kitchen and seating was most important.
I hope this helps.
Best of luck.