Use SAE 5W-40 synthetic diesel motor oil too. I do with tractors which are not near outlets and it really helps.
It won’t be cheap, but a separate deep cycle marine battery insider the van with inverter and line to the block heater. You would have to take it out and hook to a charger now and then, but it would not jeopardize your car battery. Marine catalogs have different solar powered set ups which could supply some trickle charge to battery if the expense is worth it too.
Cold weather additives should be in already, but it wouldn’t hurt to add a little more.
Lastly, because batteries also loose cranking amps in cold weather, have a cheap portable starter battery around inside where it’s warm to take with you for a little extra boost. You are right about letter glow plugs do their work…keep that up.