I am with Camlan, *inviteseller and others: say "NO!" Have your dad visit, sure, but Just Say No to buying things like lottery tickets, expensive meals, special and double food, etc. He sounds like he is well enough fixed. Let him chip in, here. You want lottery tickets, Pop? Hand over the money. Later? Then ME buying them Will Not Be Possible. Make your own vacation plans, and Just Say No. Do you know what a bundle you are saving them by being a companion/caregiver plus pet sitter -- on top of which paying for things that are in no way your responsibility? Their gardener? Really? Do you have "doormat" tattooed on your back? They are using you terribly and treating you like a non eHell approved word. I am furious on your behalf!
I know you love your father, but you truly, for your own sanity, must start saying no. PPs have given you a good idea of what rational boundaries are. Write them down and stick to them. They'll get nasty. You stay polite, yet firm. You win giant karma points for staying calm and solid and not engaging the crazies. Add those points up and do something extra special on that vacation you are now going to book for next year. Please!