Hi NWH
I guess it's your hot water heater draining the battery,
Two choices a second or third battery if you have two run in parallel if space, I ran two 110amp they only ran low if I was stationary on a few new new build sites for about 4 hours
Third best talk to Oliver at grippa, they do an option for what your asking as someone else stated, unless it was them that quoted that, they also run a on the driveway fitting service
Even though for years I fitted systems an can, I got theming so do my vans as I feel they offer a good system
Also maybe consider there split charge relay at same time, mine work very well I must say
Tony