need help figuring out how to configure my rainwater system to water my cabin with what have on hand

I am trying to make an off-grid water system for a small cabin and I am trying to put together a small water system similar in design as one that would work in a trailer or RV.  
This will be for using small amounts of water for flushing toilet/short shower/front load washing machine that uses very little water/ dishwasher that uses very little water
Here are my parameters:
WATER SOURCE: four 55-gallon barrels of reverse osmosis reject water (I am going to connect these in series so they act as one 220 gallon cistern.
PUMP: (I know most would probly say that this is not the right pump to use, but is there any janky way to jerry rig this to work?):
Option #1: Small diaphragm pump – 12V DC freshwater pump with pressure switch 1.2 GPM 110PSI (6.88 Bar) Cut off pressure.  Adjustable for RV, Camper, Marine, Boat, Lawn.
Option #2: Sump Pump?  I have several on hand but could not find out if you could use one for a pressurized system.
Pressure Switch: The pump has a pressure switch at 80-100psi.  I have a SqureD pressure switch rated at 30-50 psi.  Can I hook up this pressure switch to the pump and will this bypass the pressure switch built into the pump?
Pressure relief valve: I need to get one for my system. Is this mandatory?
Pressure tank: 3.5 gallon RO pressure tank that I think is pressurized at 6-10psi when empty; 30-40psi when full.  Ideally, I will get a larger tank but could this work in a pinch for a small system?
Delivery system: ½ inch PVC line about 25 feet to end point with only four 90 degree elbows to sink faucet.

Any advice is appreciated!!!

I still use a very similar system to what you have on hand, in fact i started out with what you have on hand, except the 12v pump was about 5 gal at 45lbs

Just siphon down to the pump, the pump should have enough of a draw to start the siphon, but i wouldn't recommend the pump be higher than the water barrels.

Send your water wherever you want it to go.

You can by bass the existing pressure switch just by shorting out the terminals on the switch- a small jumper, or just pull both wires from the switch and connect them together, then install your other pressure switch somewhere in the water  line on the outlet of the pump, so the wiring goes one pump wire to battery, one pump wire to new switch, one wire from other terminal of switch to other pole of battery.

I do not recommend running at 115lbs--most plumbing is 60 lbs max. You may be able to get a higher flow using the lower pressure setting, but don't count on that. Better to just buy a new pump that is the right pressure setting.  Are you still trying to make RO water? Or are you just inheriting these parts from an old ro system.

I have had quite good success without a pressure tank, but it does cut down on the cycling of the pump, that tank just hooks into the line as a splice between the switch and the rest of the plumbing.

I'm still using basically that same system, rain water feeding a cistern, siphoned to a 50 lbs pump, then sent everywhere, hose outlets, bathroom, kitchen, gardens, the only shortcoming is with more people comes greater demand, and at some point you might want a bigger pump, or separate systems

Filters and freezing  considerations need to be addressed as needed, diaphragms don't like leaves etc, pipes don't like to freeze.