Why not just write it down in a journal or binder book? Apps and tech fail, books don't. If you ever decide to part with the rig you can give it to the new owner as part of the history of the vehicle.
I have been doing this with my Land Rover. The primary binder - the one documenting repairs and such - is now needing to go from a 4" binder to a 6" one. Within this binder I keep the receipts of all the parts purchased, special tools purchased, gear purchased, modifications made and so on. I also print out the various tips-n-tricks used to make repairs and such that the shop manuals don't cover.
I also have a section within the binder that is a "wishlist" or "need to do" list.