Leisure Batteries

Hi everyone
Can anyone offer any advice on the issue of leisure batteries in a 2004 Auto-Sleeper Symbol campervan?
I am seeking to replace two, one which I believe came with the van at first build so 7 years old and one which is two years old. I use my van mainly for wild camping, often staying three or four nights on any wild site, usually during that period driving no more than 30-50 miles per day touring the area.
My van has an Eberspacher Airtronic D2 space heater and also an AVTEX TV/DVD recorder and both appliances have low voltage cut-outs.
My current leisure batteries are “bog standard” 85 ah but I see several companies offer “heavy duty” leisure batteries, at an increased price relative to the “bog standard” ones. I cannot easily and cheaply fit batteries larger than the current 85 ah as there just is not enough space, length and height under the drivers and passengers seats where the batteries are located.
Can anyone recommend leisure batteries from their experience that hold their voltage as high as possible for as long as possible so as to avoid issues of the heater not firing or the TV switching itself off?
Many thanks in anticipation,
Sussex

Banner Batteries seem to be the tops at the moment although last year I chose an Exide one as it had a larger capacity in the same size case as others and they also have a good reputation.
Its best to try and keep both batteries at the same capacity. If you can physically fit larger batteries do so 110 or 115 AHr will be an improvement over the 85aHr ones. Scratching my head head when the X250 came in, but yours will probably be a Boxer and the AS fitted Leisure Battery will be under the drivers seat which will limit the size you can get in the space.