24 Hour Fitness here - it's my local gym membership and is all over the western half of the US and Hawaii. Have one of those old, pre-paid memberships so continuing with them is dirt cheap. I was surprised to see several very nice new 24 Hour facilities in NYC; not sure if they acquired a local chain there?
However, I tend to select my hotel based on the quality of the gym, to the extent I can find out. I tend to travel to big cities and stay in nicer hotels so that obviously doesn't help the OP staying in small towns.
I also have the Men's Health app that has body weight workouts for when you have to workout in your hotel room. Has come in handy a few times.
[On edit] BTW, I certainly don't mean to imply that I think 24 Hour is the "Best National Gym Chain" as in the thread title. Far, far, from it - while most locations are pretty good for equipment, they're generally horrendous for locker room cleanliness (at least for the men). However, I think the OP really meant to ask about the gym chain with the widest range of locations for a frequent traveler in the US.
Last edited by Brucemcdou; Jun 19, 2015 at 6:05 pm