lymphoma in dogs

my dog has been having chemo for 2 years for lymphoma he is as bright as a button eats well no upset tummy and is always keen to go out, paying for chemo every month is very expensive how do we know if he has been wrongly diagnosed the vet tells me he really shouldnt be here now but is it lymphoma?. When we first took him to the vet he had itchy lumps and was breathless and after the chemo started he was also being treated for mange as he lost all his fur.We are frightened to stop treatment because we have been told he could die within 12 weeks without treatment and the treatment is now monthly for life we are struggling with the bill although we have insurance we have to pay 20% of the bill. your advise please.