Something I am doing right now is checking power consumptions of the multi-fuel devices in the Motorhome to check the feasibility of what could run on a battery system to cut down on gas use.
Because of the way I setup up Clarence, my Campervan conversion, I can plug the new Motorhome into the Campervan and monitor its use
Firstly monitoring the 3-way Fridge when running on 240V.
Looks like it is a fairly constant 240W (which is around 5 x the electrical consumption of my 240V Compressor Fridge in Clarence).
The effect on the Battery voltage at that current draw is quite noticable. In this 4 hour snapshot, voltage has dropped nearly 0.2V
And the key piece of info - how is it affecting the battery State of Charge? Now bearing in mine the Motorhome is actually running on the Campervans batteries, not its own, I have a 645Ah Battery Bank available. Not starting with 100% full, but how much SoC has been taken out of this large bank?
so the above shows 15% has been taken out the batteries - and this is a 645Ah bank - if I had a maybe more typical 200Ah Bank, that would be around 45% taken out of the batteries in 4 hours. And if that 200Ah bank were Lead Acid Batteries that you don't want to take below 50%, in effect you would have all but drained the available power from the battery bank in just 4 hours if running this Thetford 3-way fridge!
If you had a big solar setup, then that could sustain the fridge in the summer through a fair bit of the day, but not overnight.
I was curious if the fridge might have a duty cycle along the same lines of a compressor fridge (I think some 3-ways do?) but it doesn't look like it, so this is going to be a 240V-only-on-hookup device I think.
The Truma Ultraheat Space Heater will definately only be a 240V-mains-only (running heating on battery will be a no-go for a long time!).
The Truma Ultrastore Water Heater could be a good candidate for a decent sized battery bank though. The Campervans 10L mains heater doesn't use much power and I would expect the Ultrastore with a similar sized tank to be similar. That is a test for another day
This general info should be very useful to know how much value there will be in any upgrades to Electrical power and systems to balance cost against effectiveness