Advice on deep cycle setup?

Because space is such a premium in a conversion van, finding a place to mount them is an issue. Although they cost A LOT MORE, consider lithium-iron (LiFe) batteries. Much higher energy density (stores more for the given space)

not exactly,
its not density per space, but useable AmpHrs per 100 Amphrs of battery

put depending on the particular Lithium chemistry Li-Ion verus LifePo
the battery weighs 1/2 to 1/3 the same size Lead Acid 100 Ahr battery
it still takes up the same amount of space

but you do gain in useable ampHrs
instead of getting only 50% useable and recharge
with Li you get 80% useable and recharge
approx 40% more use per battery space used

so yes you can more use, of the amphrs stored in that available space

if 64# of lead gets 100 AmpHrs of which 50 can be safely repeatedly used (with proper recharging to full)

and 80# of Li will get you almost 200 Amphrs of which you can use 160 amphrs
but it will take up double the space of the lead battery, but give you 3x the useable number of ampHrs

it gets confusing when people swap the terms space and weight

i prefer to use space, because we only have so much space we can use for batteries, then explain that we can use more ampHrs from the Li that is in that space, ( it really doesn't contain more total ampHrs per 'packaged' space ) but you can use more of what is there without harming the Li Battery, aka effective daily cycled use is more,
but not total capacity by volume, if you had to use all the power from either one in an emergency, you would end up with the same amount of power, 12v*100amphrs is 1200 wattHrs period no matter what the chemistry
although some Li versions can deliver tremendous amounts of power without suffering peukert effect that lead acid does

sorry i did not mean to get long winded

NO, i don't have lithium, i have Lead AGM batteries, big heavy ones