Simple answer on batteries ..

A vehicle battery is designed to take a very large load over a short period ... around 60-80 amps - petrol 80 -100 amps - diesel and then get recharged at a high rate.. whatever the alternator shoves out.
A leisure battery is designed to take a much smaller load, usually a maximum of 20 amps, often much less, for a long continuous period, then to be recharged at a slow rate .. usually around 6 amps.
The internal design of the plates, and the distance between the plates is therefore totally different.
Having said that, I know many folks who have used vehicle batteries successfuly for 2-3 days, then recharged using a basic car charger and the batteries have lasted a year or so .. and they were usually second hand anyway ...
I have 2 "proper" leisure batteries in my 'van and can last 6-8 days easily, then recharge with a decent "smart" charger... both batteries are over 4 years old ....
You pays your money and you makes your choice ..

HTH