That looks pretty good to me.  
My low point drains are just capped with hand tight fittings.  When I want to drain, I just uncap them.  I am not confident that all the water drains out though, due to bends in the pipes.  Caps would also defeat your idea to keep all pipes outside the van empty.  The half or 1/4 turn ball valve idea should be good.
An accumulator tank is just small pressure tank to help even out the pulses from the water pump and reduce the amount of cycling the pump does at low flows.  It gives you a bit more pressurized volume, so you could fill a glass of water or flush a toilet (in a trailer) without the pump coming on (nice at night).  It can be placed anywhere in the cold water line, it is not necessary to have it near the pump.
It is absolutely not essential, but could be nice to have.  Installing one is on my to do list. :)  I'd say better than 99% of RVs do not have them.
Regarding PEX, I think that's what most RVs are using now, mine's older & uses gray tubing.  I've heard good things about sharkbite connectors as long as there's no strain on them, but have not used them personally.  Hopefully someone else will chime in on this one.
CC