8 mpg pulling my 28 ft 5th wheel
 

I have a 95 chevy 1500 6.5. good power till about 60. poor fuel millage think the timing could be off would this effect this? the inj timing at Idle is 6.5-6.9

Look at your GVWR load on your door ie. 5630 lbs for example. most half ton trucks have 3.08 gears fitted which are meant to give you good fuel economy, the 3/4 ton and 1 ton trucks go upto 7200lbs 8200lbs etc different gearing load capacity's. Go too a 5th wheel site.... and they usually have graphs that will let you see in a instant what load capacity you are okay with with a 1/2 ton truck.:c: I suspect the fuel economy issue will be to do with your rear axle.

X2 My 3/4 ton is very low geared like top end 80ish but I can idle with a 27' goose neck with 7 rows of hay all day long across any field oh and with the A/C on Max:w2: Oh and yes I have the NV4500

Another factor for the low geared trucks at highway speeds is that the stock turbo lays down above 2400 rpm so to save the engine and fuel you sometimes can't travel over 60 mph no matter the posted limit.

The 1/2 ton is likely just plain overloaded with your trailer unless it is an alumilite with empty tanks.

Remember these are light duty diesels built for economy not medium duty work trucks like the Cummins and IH ford uses,the worst cummins and powerstroke out muscle or trucks and only go up from, there.

The little 6.5 is only 180 hp (134 kW) @ 3,400 rpm / 360 lb·ft (488 N·m) @ 1,700 rpm
there was a special built marine engine that exceeded these specs but is not offered for trucks.

Cummins:
5.9L Cummins 24v Specs

Ford/IH 7.3:
Ford Power Stroke engine - Wikipedia, the free encyclopedia