would it not be better just to use jump leads from the leisure battery.
in my truck i use same size as engine batteries for leisure use plus a couple of bigger ones to make the leisure size bigger.
have on the odd occasion had to put the smaller leisures on for the truck.
how do you normally charge the leisure batts .?
simple electric systems rule for me .
You could----- but it depends on the size of inverter and battery charger------ and if your leisure batteries would run long enough to put enough in the engine battery to start it-------- if it is a start/charge battery charger it might give enough of a quick burst to start the engine, but they do draw a lot to run!!!!