I don't see a big problem - I'll come back to that - but I don't see the point. If I am reading correctly, you re going to have two batteries, one on the tongue and one (the Jackerie in the rear compartment) serving as the TM main battery. You are going to wire them in parallel - that is, plus to plus, and minus to minus. The fact that you will use long round-about wires to do it doesn't change anything. With the batteries wired in parallel, they behave as a single battery. Your battery tender will charge both of them when you are camped and it is plugged in. And the 12-volt line from the truck will charge both of them, and power the TM loads (appliances in the camper, for instance) including the breakaway connection to the brakes, when you are driving. So I don't see that you have gained anything except a small increase in total battery capacity. The breakway function still works as desired, of course, powered by both the small battery and the Jackerie.
As to reasons not to do it? It is generally considered unwise (though not catastrophic) to connect two batteries of the same chemistry (lead-acid, for example) but different capacity together. Your TM battery problem has a capacity of 75 amp-hours or more, while the small battery has a capacity of 5 or 10 amp-hours. Not a good balance. If the two batteries have different chemistry, such as lead-acid for one and the lithium in the Jackery for the other, it is even more unwise.
The kit you are looking at is intended for utility trailers that have no main battery. In that case, the small battery is the only source of power for the brakes in the event of a breakaway.
And I've lost how the Jackery is involved. How does that fit into your plan? Are you thinking of three batteries - the TM main battery, the small tongue battery, and the Jackery? Maybe don't do that.
Bill
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