Where is the thermostat set on the water heater? It could be set way down - if so, try turning it up some. Are any of the pipes exposed? Can you put some insulation around any of them? A really long run can allow the water to cool off some, especially if they run through unheated areas (not too much of a problem this time of the year most places, though). If the old house has a bunch of air leaks and they go over the pipes, this can make it much worse and can lead to frozen pipes in the winter. What kind of a valve do you have in the tub? Some single handle valves have a stop to prevent it from turning too far and if not set right can keep it from getting as hot as you want. This might be able to be fixed with no money at all.