Er, I think there has been a misunderstanding here. A 100Ah battery means that it should be able to provide the equivalent of 100A for 1 hour. However car batteries are specified on a 10 hour rate and leisure batteries usually at a 20 hour rate.
Ergo, a 100Ah car battery should be able to supply 10A for 10 hours, or the other way 200A for ½ hour. A 100Ah leisure battery should supply 5A for 20 hours. In practice a fully charged battery might start off looking like it will make it but they rarely do!
In terms of a mover, you might be using it for, say, 10-15 mins at most, so even if your unit is pulling 30 per side, a 75Ah battery should have enough (theoretical) oomph to move the van for 1¼hrs. Thus 15 mins will only deplete it by 20% - which is nothing.