Never forget to put an override relay (normal open, closing when the engine has run for a set amount of time or when manually bypassed) and a fuse in the path though (and that also applies for campers). There are a number of things to consider, and if done badly, also serious risks:

- you don't want a permanent connection between the main and aux battery to avoid accidentally sucking both batteries dry on the aux-battery side, and to avoid the starter overloading the cable between the two batteries

- you don't want to risk an empty aux battery charging itself on the main battery and engine with more current than the cable supports, hence the fuse

- you do want to be able to connect both batteries in a scenario where you accidentally drained the main battery (e.g. a light left on) to "self-start"