The 350 is a pretty good engine. If it has worked for you, replacing it with the same thing would probably be easiest and cheapest. However, if you need more power, you can see what can be fitted into the vehicle. Room in the engine compartment, computer interconnections with the vehicle, cooling requirements, mounting points and the power handling capability of the rest of the drive train all need to be considered.
If your unit is 24 feet or longer, I would see about a larger (400 or 454) engine. The 350 would be over worked and would give you lower fuel economy(maybe 6-8 mpg instead of 10). The 454 would give you much more power but would over kill in a 22 foot or shorter. So if you are somewhere in that middle range go for the 400. My Eldorado was a ford with a 302 but it was 19 by 7 1/2 with a 1 ton chassi under it. It only got 9 mpg.
okay, here's a smallblock lesson. a 350 and a 400 are basically the same motor.in fact everything chevy v8 260-400 are the same. the only difference is in the power combos. most 400's have the same or less hprsepower than a stock 350.it varies according to the specific engine and application. example 78 camaro -LT v8 350- 175 hp, same car inz28 model 270 hp. you may want to buy an upgraded 350 from Gm or a good performance shop. there are pretty good prices on mild 350's..i would think 300hp would move you pretty good. if you go up to a bigblock, you will be spending alot more money on suspension changes, transmission,ect
The Corvett had a 350HP 350 in the early 70's but you need torque when towing and you get more torque out of a larger engine. I would upgrade if possible to a newer truck with the overdrive tranny and the larger engine with fuel injection just to get the better fuel economy(15mpg vs. 10mpg based on 2500 series). Or maybe go with an older dodge deisel.
If you go with a 350 make sure it is the four bolt main engine which is probably what you have. The truck 350 four bold main developed more torque at a lower RPM than the standard two bolt main car engine.