To us one of the attractions to having a TT is the ability to bring along our own food. This was especially true when the kids were still with us. You know what they will eat, and we ourselves knew what we'd be having ahead of time (my wife is the consummate meal planner). Saves a lot of money vs. eating out, money we'd rather use to see attractions and so forth. Replenishing on a long trip is done at a supermarket wherever we happen to be.
With that said; breakfast and dinners are almost exclusively in the TT. I cook breakfast, DW cooks dinner. Lunch? Depends on the situation. If we were touring, sightseeing, etc. and away from the trailer, we'd purchase. Ditto on a long driving day, although we often would stop and make our own lunches if we found a nice place to pull off the road for a while. We always have snacks with us in a cooler to take care of "the munchies." And yes, we have stumbled on to some terrific eateries in our travels when we really didn't feel like making our own lunch.
DW has but one rule - even on a brief camping trip, say a long weekend, we eat at least one dinner out. As many have said, you ARE on vacation. Gotta treat yourself once in a while.