hi could anyone explain to me how to connect two batteries to increase the amps and is it possible to use seven foot of cable as i am thinking of connecting another battery under the front seat on the opposite side to the other many thanks
You don't state the purpose of wanting to join two batteries to have more amps available, but this is the way to do it.
If you connect the positive terminal of one battery to the positive terminal of other battery, then the negative terminal of one battery to the negative terminal of the other, you will have two batteries connected in parallel. If both batteries are 110 amp you will have access to 220 amps if both batteries are fully charged. It follows that a pretty hefty cable would be needed to connect the two batteries and the cable from the battery to the 12Vdc input to the caravan system would also have to be beefed up. You may also find that the battery charging system cannot cope with the requirements of two batteries.
If you mount another battery in the opposite locker, it would have to be well bolted down and, ideally, in its own box to prevent it being thrown around and to prevent battery acid being spilt on the contents of the locker.
I notice that you have headed the thread "Caravan microwave". Is it your intention to run the microwave of the 12Vdc supply through an invertor which changes 12Vdc to 230Vac? It would not be a good idea as microwaves take roughly twice the current that the rating would indicate. For example, an 800 watt microwave would take roughly the current for 1600 watts. Providing 230Vac for this purpose through an invertor would take roughly 133 amps (1600 divided by 12) from the battery possibly more to account for inefficiencies in the invertor. 1600 watt invertors are getting into industrial size and are correspondingly extremely expensive. It really is a non-starter if you don't mind me saying so. However, if it is not your intention to run the microwave off the caravan battery, ignore my last paragraph. With a drain on the battery/ies of 133 Amps, the battery would be empty in about 1-1.5hours (once the battery capacity gets down to about 50%, it is reckoned to be empty). If you do intend to run the microwave where there is no EHU, you would be better to purchase a small generator as as a safer way of doing it but, again, a 1600w generator is not a cheap item as you would probably have to go far a 2kw genny.