Some many years ago I had a vehicle with a "difficult" battery location, and of course the one that's hard to jumper always needs it the most. At the time I found a "terminal block" that I mounted in the engine compartment where it was easy to get to, and just ran a long wire back to the battery. You only need the "Pos" terminal for most vehicles, since the whole frame is the "Neg" side of the circuit.
Been looking for another one of those terminals for my camper for about 3 years now, and I guess they just don't make them anymore. The camper is just about 2.4 feet too far back for my 20 ft jumpers to reach the battery on the tow truck, and I have to pull the battery at least half way out of the box to get the clamps on, so an "auxiliary" terminal would be really nice.
Of course, if the dummy who plans my trips would remember to charge the camper battery before we get to the camp, it would help...