DX Engine Acceptable RPM / Ve

I have a DX engine in my T25 campervan and using a semi suitable 4 speed gearbox.

What RPM is it OK to leave it in during lower gear hill climbs? And also what RPM for a DX is best at cruising speeds?

Thanks

Max torque is about 3500 rpm, any thing over 5000 rpm and the power drops off.

They will sit at 4000 rpm all day long but the lower the rpm the less fuel you will use.