Kim.
i. You want a "leisure type" battery, not a vehicle type.
2. If you have a motor mover fitted, then anything in the range 85AHr to 120AHr would be fine. If no mover, then, unless you are a keen "Rallyer",(ie use the van without hook-up for several days) then 65AHr to 85AHr would suit.
3. Batteries vary in dimensions considerably, dependant on make/construction, particularly in height and the location of
connection terminals. Check the dimensions of your battery box, and battery very carefully before parting with your money!
Have you checked the charger operation when you are on electric hook-up? The charger output should put a minimum of 13.7 volts across the battery terminals (ie float charging) when operating. If this is not the case then do not be too quick in condeming your battery. You may have a fault with your 'van charger!
Hope this is helpful
Dave