So 5 hours is way too fast, something is defiantly wrong...
Lets start with what the van is and has.
Is the van a home build or a real Sportsmobile?
You said it was a 7.3 diesel so has two starting batteries. One under the hood and one on the frame by the side doors.
Does it have a house battery or two?
The inverter is not the thing that separates the batteries from house and starting. For that is it either a relay/separator or an isolator. either of these devices charge the house battery when the van is running but divides the battery banks when it is off. It may have an manual switch to connect them possibly.
The best thing you could do it check all of the batteries.
If they are wet cell batteries pull them out check the water/acid level and add distilled water if they need it. After they have been fully charged check the specific gravity of each cell
Any type of battery charge them with a good three step charger. After they have charged (sometimes this can take up to 24 hours) I would take them to an auto parts store or repair shop to load test them. If they pass the load test recharge them and put them back in the van, if not replace them. The two starting batteries should always be done as a matching pair. If one is bad it will drain the other one.
If the batteries all test ok then you have to see if they are charging while you drive...
That should get you a start...
There are many really great threads on this forum about batteries, charging and things like this.
Use the search and read up about it..
Here is one to get you started...
http://sportsmobileforum.com/viewtopic. ... it=battery
...Jamie
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