As to the hot water heater, there are the factors of cost involved.
You said your daily consumption was about 500w, so you splurge and get a 1000w inverter, just to make sure you've got enough. Now you need your 50/mm2 (1/0 AWG)wire to connect it up to the 12v battery system in your van. Then you need a 120a breaker and all the solar you can physically fit on the roof. Not horrible
OK, so now you buy the shiny on-demand electric hot water heater and it only draws 1200w of power, but you still need the 500w for everything else that's running, so now you have to spend the money for a 2500w or 3000w inverter. Ouch, but you can swallow that if you have to. Now you have to sell a kidney for the 300a fuse and the 250/mm2 (500kcmill) copper wire and lugs to feed it! That's a 20-ish mm diameter wire to run! OUCH!!!
You would be MUCH better off getting one of the propane on-demand heaters and just worry about powering a water pump rather than trying to wire a 3000w inverter into a 12v system.
As to the panel figuring, with your area you're only going to get half the rated power on average at best. Add in that you'll only get about 3-4 hours of good sun and you'll lose about 20% by not being able to aim them toward the sun and calculate with that. The math should go something like (Panel Watts X 1/2 usable X 80% sun angle X 4 hours = Watts produced per day) so go from there. You're biggest limit to how much panel you can use is the physics of how many panels you can physically bolt to the roof. You really can't generate too much power so go hog wild.
Since you're starting from scratch, jump straight to Lithium if you can. You'll get the best usable watt/hours per kilogram with those over any other type.
You'll need the correct type of DC-DC charger to get your alternator to talk to a Lithium battery so you can get the proper voltages to it.
I'm sure there's more but that gets you started. First thing: Power Audit! Second thing: Measuring tape and panel dimensions to figure out how much you can stick up there. 3rd thing: rough plan your system. 4th thing: Come let us take a look to figure out what you missed (because we've ALL missed something our first couple go-rounds.