This is almost always a problem with the cascades plastic cold water inlet fitting, it incorporates a non-return valve and can stick and jam so either stopping the tank filling or reducing pressure so it eventually fillls put little left to get hot water to the taps. If so flow too the hots taps will be far reduced to a trickle?
A certain check is to remove cascades drain bung and then get someone to switch pump on, water should now gush out something like a foot out from the van, so stand to one side!
If it does not then black plastic inlet fittings need replacing, if though the older white ones, unscrewing the hose connection cap, cleaning and reseating what must be a perfectly round tiny 'o' ring usually cure the problem more easily.
If though it does gush out, another possible is the narrow passage upto the tank is blocked with lime scale, or far rarer still, a blockage in the pipework to the taps
