So what you are worried about is that DH might have to go to hospital, leaving you alone at the site and unable to drive yourself home if the site were all booked up and you had to leave? Your best action is to be prepared for just that contingency, then you won't be so anxious.
First thing to do, as you are at the caravan site, is to go to the site owner and tell them the situation. There are several ways in which they could be helpful to you - you are not all alone in this, so don't feel that it is all on your shoulders.
Ask what the arrangements are in the case of an emergency, how do you phone an ambulance,and what is the exact address and phone number of the site. Keep this with you so that you can contact the ambulance quickly if you need to, and tell them exactly where to come. They will be able to put that address into their GPS. Have an overnight bag ready packed for him with essentials and medicines and so on, plus a small amount of money and the address and phone number of the site.
If you go with him to the hospital, you will need a way to get back to the caravan. Ask the owner or the site office about taxis and public transport - what would they advise for getting back to the site, possibly in the middle of the night? Keep the taxi phone number to hand in your purse, and enough money to buy you a snack at the hospital shop and pay your fare home - plus the keys to the caravan!
While speaking to the owner or site office, also ask whether anyone is booked into your pitch after your holiday ends, and whether it might be possible to extend your stay if necessary.
Think about how you would get home if DH has a lengthy stay in hospital. Would your insurance pay to get you home? Could a family member or neighbour come and fetch you? How close is the nearest train station, and could you get a taxi to it? If you have a lot of bits and pieces of stuff with you, and no family or neighbour to drive you, would someone from the site drive you home in your car, with all your luggage, and get a train or bus back (with you paying?)
Have your plans made, all the phone numbers you might need noted down and kept safe, DH's hospital bag packed and ready, your allies prepared to help you if it is needed, and then - stop worrying about the faint possibility of having to put it all into practice and ENJOY YOUR HOLIDAY WITH YOUR HUSBAND. He has enough to worry about with his health, without having you anxious all the time.