As discussed the hot water flow is poor compared to the cold water flow. There are some things to check. I have attached a copy of the GSWH-2 technical manual to this email, which may also help.
The cold water inlet has 2 components a knob to adjust the flow rate and also a filter just inside the inlet. Make sure the white knob is opened to allow flow and disconnect the water pipe to remove the filter, check for debris and clean if required.
A tempering valve may be fitted (Pictured Below) - this will be located on the water pipe under the van after the kitchen sink before the shower. ADVISE IF FOUND!
Also follow your plumbing lines from the hot water outlet, check for any questionable pipework tight turns or kinks. Feel free to supply photos that
may help diagnose the issue.