I have accidentally accumulated spare parts and loads of supplies in the past 7+ years of 24/7 in this little old rig.
Mostly leftovers, like leftover brake fluid, leftover oil and so on. I have extra lights. When I had to buy a turn signal light, I bought 2, ditto for brake light. I have spare interior lights and spare fuses.
Once a year I assemble all the stuff needed to give the generator a good maintenance, then I typically pay someone else to do it.
I have the usual duck tape, wire ties, gear ties, WD-40, Silicone spray, screws, Marine-Tex, JB Underwater weld, 5200 calking, 2 part epoxy, super glue, outdoor double stick tape, assorted bug sprays. Not exactly spare parts but when you need it, you need it badly. A heavy duty wire tie (or several together) can fill in for a hose clamp until you can get one.
I accidentally have a spare starter! I wish I could sell that.
If I got rid of all my tools and repair supplies, I could have a nice expansive wardrobe. LOL!
I have a box full of bicycle stuff, but mostly because I do reviews of new products, so they are always sending me stuff to test.
I used to have a fix-a-flat for a big truck tire and I used that one day when I woke up to a flat tire in the campground. Filled the flat, then drove to a tire store. I should replace that, it sure was handy. I have fix-a-flat for the bicycle tire.
I used to have gallons of transmission fluid, but thankfully got that problem solved. Turned out I needed a bigger torque converter and that solved the transmission throwing up when it got warm.
If I were you, I wouldn't buy any spare parts. I would wait and buy as needed. I have Amazon prime, so they deliver in 1-2 days to my campsite. Their rates are highly competitive.
My problem is I buy extra. I need one light, so I buy two. I need three screws and buy five. I need super glue, so I buy 4. I think it's because I used to live on boats in far flung ports and one gets used to making do with nothing, and carrying around useful supplies.
Last time my transmission threw up, I was able to coast into a camp with campers pushing me the final 100 yards in the middle of nowhere. I kid you not! It was hotter than hell and I was pooped out. We ordered more transmission fluid and paid for a week's stay. Finally drove out of there with the new fluid and found an awesome transmission shop that diagnosed the problem. Had to wait another two weeks for an appointment, but he did it all in one day. No problems since then.