If I was looking at doing decent a power setup for a big lap etc, I would be looking at going one of these
Tesla battery modules. For comparison's sake, that produces about the same as 4 x 12v 120Ah lithium (liFePo4) batteries, but at half of their weight, a third of their volume and a fair bit cheaper! Being Tesla modules too, it contains the best quality lithium cells (LiIon) that are currently being produced, and are designed for a much harder life than they are going to get in an RV.
An issue though is that it's 24v as opposed to 12v, that's pretty easily and cheaply dealt with it you are doing a setup from scratch for most things, but not so being able to charge from vehicles with a 12v alternator. But like most problems, they can be viewed as an opportunity, if you added an additional 24v alternator then you could make use of the batteries rapid charging capability. Say if you added a 100A 24V alternator, you could then charge the batteries at a 12v equivalent of 200A, this would mean you could go from low batteries to half full in just over an hr.
The other advantages of a setup like this are I think pretty significant too, you could do away with an LPG set up by using an inverter to power 240v induction cooktops and a 240v instant hot water heater, that alone would be pretty significant cost as well as weight savings. You could also then use 240v airconditioners and fridges which are both generally quite a bit cheaper as well as more efficient due to the economies of scale involved with their design and construction.
It might not stack up as well if you are looking to retrofit, but a new build I think would be well worth it.