@Alexandra07 The DFDS ferries from Dover to Calais and Dunkirk are car ferries. There is no option for foot passengers, so everyone has to be in the car/van/camper. What you could do is ask about priority boarding and being allocated an "assistance" parking space next to the lifts, so that you don't have to walk too far on the car deck.
Regarding taking food into the EU. You might be checked at the French border (which is actually at Dover) and again, you might be checked at Dunkirk. As others have already posted, you might have to throw away any ham sandwiches etc. that they find but you'd be in bigger trouble if you have a cool box full of bacon, cheese, pork pies, frozen lasagne and raw meat.
The food on these particular ferries is actually quite good. The fish and chips are excellent and reasonably priced. There was indeed a time (after the first Covid wave) when there were very few passengers and freight lorries and DFDS were offering free meals, but that has ended.
Otherwise, "emergency food" - pizzas, milk, cooked meat, bread etc. - can be bought at the mini-supermarkets at the motorway services and larger petrol stations, even on public holidays.