So glad you're finding the website to be helpful. And sorry to learn you're pinned in a difficult spot.
What you are asking seems completely reasonable. And you are wise to keep the receipts and be organized about the money spent in carrying out your duties as your father's appointed agent.
But the law doesn't always automatically support the reasonable approach. The law does not automatically entitle you to be paid.
Normally, in family situations where the attorney-in-fact's duties are simple, no payment is provided. However, if an attorney-in-fact has to run a business or manage complicated financial affairs, then payment may be appropriate.
However, if your dad agrees you should be paid, that should be clearly stated in the power of attorney document or in a brief contract or agreement that is separate from the POA. Many people prefer keeping the two separate, since that ensures that the financial arrangement will be kept private, whereas the power of attorney is intended to be shown to various other people.
Since your dad entrusted you to act on his behalf, he may also well agree that you should be compensated for your travel and hotel expenses. Showing him that you keep good records, and assuring him that you want compensation only for expenses incurred in doing your duty might help motivate him to make the written agreement to reimburse you.