So why when trailer b?5ased, to much amp draw?
Sounds like you're thinking electric, whereas most vehicle heat exchangers use diesel (Espar). Heating water with electricity is amp-intensive, so it really depends on how much water you need to heat. My trailer, which is primarily a toad for my RV (C5500 Topkick build), uses a 2gal mini-tank electric water heater for the wash basin. It is the least amp-intensive electric water heater you'll find, at about 17.5 amps, 110VAC. The "on-demand" electric heaters will require about 27 amps at 110VAC. Either means at least a 3000 watt inverter with some decent battery to back it up (I have 440ah in the trailer, along with 600watts of solar on the roof). If you've already got propane on-board for cooking, it will be your best bet for heating water. Or you can run a generator and not worry about the electric.
x2 on the two tank setup. I only have one in the trailer, but there are three in my RV. I wanted to be able to isolate them when needed, so they all have their own fill ports and vents (very important and often forgotton on custom tanks). The outlets all have a ball valve, and feed a common manifold, which then feeds the pump. After the pump you've got a backflow preventer, city water fill, pressure regulator, and hot and cold distribution via a "T". Most RV and marine pumps do best with fittings from RV or trailer supply stores, rather than the standard PEX or tubing stuff you find at Home Depot or Lowes. The same is probably true of the poly fittings on your tanks. All the rest of my fittings were sourced through standard plumbing supply houses. And I used PEX for everything but the fill hoses, rather than the traditional poly tubing.