Not sure how open you are to suggestions, but this might be an option based on your description:
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So let me explain my reasoning:
You tow a boat and like to fish.
A truck cap gives you dry and secure storage for anything you take with you or leave behind when you take off with the boat. Heading out to the lake you just toss it in and close the door. No worry about tie-ing things down or loosing them on the way there. You can run into the gas station to pay, buy snacks or even just to pee without worrying if your new chainsaw or cooler is still going to be there when you come out.
It also gives you a nice place to spend the night on longer fishing trips that's safe, off the ground and out of the rain. Depending on how much work you want to do, it can be downright "homey":
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here's a few more of that one just because I think he has an awesome setup:
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Not so much the dodge, but I never thought a plain old aluminum shell could be sooo useful or cool! If I was younger and still adventurous enough to venture into the wild with less than a 35 foot trailer, that would be what I would build.
You can get them fitting the style of the truck cab if you want:
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Canoes? No problem:
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Lights are no problem. Many expedition rigs do the same thing:
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Awesome idea if you like to competition barbecue:
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Insert your boat and a lake in this pic and would be a fishing trip to remember:
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Doors, gates, flip tops, side storage, windows, etc. The choices are endless.
If you are a metal worker it's even easier to get some serious weight capability up there, even on a fiberglass shell. Just make four struts for the inside of the shell (tie the tops together with a hoop or bars for lateral strength), build a top rack the size you want with "feet" that match up to the internal struts and then bolt 'em together with the fiberglass sandwiched in between. . Heck, you could have multiple top racks you could swap around depending on what you wanted: a full ladder sized rack for long loads and swap on a smaller "expedition" style rack for other times. A couple flat plates would work to cover the holes when you didn't want a rack up there at all. Your imagination is the only limit if you can bend and burn.
Then, you could install a few items back there that would make dual batteries and alternators worthwhile.
Just a few thoughts you might want to consider....