Firstly - apologies for the delay in response, we ended up taking Friday off as well to have a 'long weekend' off!
It seems like you have your work cut out for you. A lot of custom stuff with the water side of things and I like how well thought out this is. Will you be using flexible hosing or rigid piping for the water lines? Aside from this, it seems like you have a very well thought out idea on how things are going to be. I like it!
Yes, very good point. Trial and error will quickly find the sweet spot and you'll be able to use the Shurflo on its own.
Now, to give you a bit of insight into unit operation (for anyone reading with a similar HWS this holds true too):
The HOTTAP flow rate is between 4LPM to 6LPM (will be a bit of deviation here and there as with anything). So, if I'm supplying 4LPM of water to my HOTTAP (or other portable water heater) then I'll be able to happily use it on the low flow setting, pushing up to about the mid flow setting. Once I go higher my unit will cut out as it is being starved. It is expecting a higher flow and it isn't getting it. This is why our supplied pump meets the max flow of the unit. The HOTTAP (and other portable systems) have a flow regulator on the water inlet - reducing anything above 6LPM down to a maximum of 6LPM (so mains won't give you a 'fuller' shower over a 18LPM pump over a 6LPM pump).
So, this is where the sweet spot comes into play. If you use the 6LPM setting, you won't want to drop too much below that supply flow otherwise the HOTTAP will cut out. Having an extra 4LPM (or so) to play with means that this should actually be possible. I always give a word of caution beforehand so that there is always room for recovery. Though, it seems like you know what's up!
The Jacket is awesome for general protection. From rocks getting flicked up to rain, it is actually very decent at what it does. I don't doubt that we have had people run the Jacket through all sorts and we actually haven't had anyone come back and say the Jacket didn't do what they needed - I'm sure this gives an idea of what to expect.
Now, in terms of full protection - I would make a sealed cover/ case for it. A lady once told me that the 'notorious' red dust even crept up into the floor of her camper. So, the Jacket (having an opening in the back to slide of the unit) won't keep this out. At the same time, it won't keep it 100% water proof so you wouldn't want to get the HOTTAP dipped in too much water. Though, some water splashing shouldn't be too much of an issue as the bottom panel is sealed with a water proof zipper. Have a look on our site for some of the images of how it wraps around - specifically the back opening. I'm sure you will be able to gauge how much cover this would provide for what you are thinking.
The battery box is on the bottom RH corner of the unit, main control board on the bottom LH of the unit, and electrical wiring is in between. So, pretty much all electrics are on the bottom of the unit. We have added some form of water resistance to the CB, but it is not completely water proof. We actually have stock of spare parts for everything in the HOTTAP so if anything does fluff up, we got you covered. A splash of water here or there should be OK, but a splash is relative. My a lot could be your little, and vice versa. Haha! I'm sure everything will be ok though. Haven't had anyone give us a call for a submerged unit yet.
Hrm, weird. Can you PM me first and that way I'll PM you back. I might honestly not know what I am doing in terms of messaging people initially - haha!