There are many disabled access informational websites available. Most (if not nearly all) national recreational areas would have disabled access available. In addition to wheelchairs, there are also all terrain power chairs which have many drive trains but will go easily wherever you want to go and many are available for rent.
If you have a particular town in mind, I would encourage you to contact the tourism office which will be able to assist you with any disabled access questions you might have. I traveled to over a dozen state and federal parks last summer and all of them had disabled access, though nature limits certain areas.
If you are disabled, you and anyone who travels with you, can get a lifetime pass to national parks. The card is issued to the disabled person and they can use it for several people. I had 15 people in one vehicle.
I hope you have a fun and memorable vacation. Please be mindful of the heat and stay well hydrated.