Change or adapt waste tank

Harcourt3

We have a Chausson Allegro and the only complaint about it is the grey waste tank and emptying it. The tank is fixed in the centre underneath with only a 4" circular outlet in the centre. Hence we always hav to park directly over the grey waste disposal
and pull the handle or OH has to slide a washing up bowl (small one) under to drain a bit at a time. Can anyone recommend somewhwere that we can take the van ( South of M25) to have a new tank installed or the current one adapted. DIY is beyond our capabilities.
Any help would be appreciated.