Wow, I feel like someone needs to back your husband just a little. I'm sorry you have to be going through this and it sounds like your "gut" is telling you what could very well be happening here, but if I may, let me tell you about my experience.
A few years ago we were in a similar position. My husband is a brick layer and there wasn't any decent work where we live because it's a "who you know" sort of trade and no one knew him where we lived.
He moved to his hometown 4 hours away where his name is household so that he could get a better job. He worked sometimes 12 hours a day, in the heat, and sent most of his paycheck home, keeping just enough for food and an occasional golf game.
He drove home every other weekend, giving him a weekend to relax, and then we moved down there with him after a couple of months. Although there were times I could have suspected something, I had faith in my husband, and as it turned out, he was just much too busy to be doing anything shady.
You're married to your high school sweetheart, which in my experience with friends who married right out of highschool, there's not a lot of time to explore other things, ie. other people, the single life, and what not. This can lead people to feel resentful after so many years, but not always.
You do need to talk to him, but please, for your family, give him the benefit of doubt and really listen to him too. Just tell him everything you've told all of us, without accusing him of anything. Tell him how hard it is for you because you love him so much and you're afraid of losing him. And make sure he knows you're not coming down on him - men will get defensive pretty quickly - even when they've done nothing wrong!
If you attack him, it won't matter if he's up to something or not. Your marriage could blow up for nothing. Love is patient and kind, so that much you've got right. Until you have proof of wrong doing, don't risk losing those 11 years over your suspicions, however right they might turn out to be.
Someone suggested a surprise visit and I think that's a great idea! If you can manage someone to watch your kids for one night, or even a day, take of a day from work or go on a weekend, whatever you can manage. Don't "spy" on him but dropping in unannounced will give you a more accurate feel for what's going on. You can see how he's living and you can gauge his reaction to you showing up unexpectedly.
Just don't call a divorce lawyer before the two of you have really had a chance to get through this.