+1 on the LiPo batteries. WAY smaller and lighter. Really robust too. You could put two LiPo batteries in a fire bag for less than the cost of a decent deep cycle. Two 7.4 volt LiPos in series will start my truck no problem. 5.2 amp hour, 14.8v nominal, about $45.
Why not get like a proper tongue box from Harbor Freight? They're cheap, light, and designed for this.
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